Rare Earth Magnet and Method for Manufacturing Same

ABSTRACT

The object of the present invention is to provide a rare-earth magnet having a sufficient corrosion resistance, and a method of manufacturing the same. The rare-earth element in accordance with a preferred embodiment comprises a magnet body containing a rare-earth element, and a protective layer formed on a surface of the magnet body. The protective layer in accordance with a preferred embodiment includes a first layer covering the magnet body and containing a rare-earth element, and a second layer covering the first layer and containing substantially no rare-earth element. Another protective layer in accordance with a preferred embodiment comprises an inner protective layer and an outer protective layer successively from the magnet body side. The outer protective layer is any of an oxide layer, a resin layer, a metal salt layer, and a layer containing an organic-inorganic hybrid compound.

TECHNICAL FIELD

The present invention relates to a rare-earth magnet, a rare-earthmagnet having a surface formed with a protective layer in particular,and a method of manufacturing the same.

BACKGROUND ART

As permanent magnets exhibiting a high energy product of 25 MGOe orgreater, so-called rare-earth magnets (R—Fe—B magnets, where R is arare-earth element such as neodymium as in the following) have beendeveloped. As such rare-earth magnets, for example, Patent Documents 1and 2 disclose one formed by sintering and one formed by rapid cooling,respectively.

Though the rare-earth magnets exhibit a high energy product, theircorrosion resistance is relatively low since they contain a rare-earthelement and iron which are relatively easy to oxidize as mainingredients.

For ameliorating the corrosion resistance of such rare-earth magnets, ithas been proposed to form a protective layer. Among them, PatentDocument 3 proposes to form a protective layer by heating a rare-earthmagnet at 200 to 500° C. in an oxidizing atmosphere.

Patent Document 1: Japanese Patent Application Laid-Open No. SHO59-46008Patent Document 2: Japanese Patent Application Laid-Open No. SHO 60-9852Patent Document 3: Japanese Patent Application Laid-Open No. HEI5-226129

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

Though the above-mentioned Patent Document 3 proposes to form aprotective layer at a specific temperature in an oxidizing atmosphere,there have been many cases where even such a method cannotsatisfactorily form a protective layer which can sufficiently preventrare-earth magnets from corroding. Therefore, thus obtained rare-earthmagnets have still been hard to fully prevent powdering and weight lossfrom occurring in corrosion tests.

In view of such circumstances, it is an object of the present inventionto provide a rare-earth magnet having a sufficient corrosion resistance,and a method of manufacturing the same.

Means for Solving Problem

The inventors conducted diligent studies in order to achieve theabove-mentioned object and, as a result, have found that a corrosionresistance superior to that conventionally available is obtained when aplurality of layers different from each other in terms of composition orconstituent material are formed on a surface of a magnet body, therebycompleting the present invention.

Namely, the rare-earth magnet of the present invention comprises amagnet body containing a rare-earth element, and a protective layerformed on a surface of the magnet body; the protective layer having afirst layer covering the magnet body and containing a rare-earthelement, and a second layer covering the first layer and containingsubstantially no rare-earth element.

The inventors presume that the following is a reason why the rare-earthmagnet having the structure mentioned above has a sufficient corrosionresistance. A rare-earth magnet contains a rare-earth element as itsconstituent element. This rare-earth element is very easy to oxidize andis likely to be eluted into acidic solutions. In the rare-earth magnetof the present invention, by contrast, the protective layer has a firstlayer covering the magnet body and containing a rare-earth element, anda second layer covering the first layer and containing substantially norare-earth element. It seems that, since a surface of the rare-earthmagnet is thus covered with the second layer containing substantially norare-earth element, the stability of the protective layer improves,thereby ameliorating the corrosion resistance. It also seems that thusconfigured protective layer becomes dense in structure, therebyimproving the stability of the protective layer and ameliorating thecorrosion resistance.

Preferably, in the rare-earth magnet of the present invention, theprotective layer is formed by heat-treating the magnet body in anoxidizing atmosphere containing an oxidizing gas while adjusting atleast one condition of a partial pressure of the oxidizing gas, atreatment temperature, and a treatment time such as to have the firstlayer covering the magnet body and containing a rare-earth element, andthe second layer covering the first layer and containing substantiallyno rare-earth element.

The rare-earth magnet of the present invention may comprise a magnetbody containing a rare-earth element, and a protective layer formed on asurface of the magnet body; the protective layer having a first layercovering the magnet body and containing a rare-earth element, and asecond layer covering the first layer and containing a rare-earthelement by an amount smaller than that in the first layer.

The inventors presume that the following is a reason why the rare-earthmagnet having the structure mentioned above has a sufficient corrosionresistance. A rare-earth magnet contains a rare-earth element as itsconstituent element. This rare-earth element is very easy to oxidize andis likely to be eluted into acidic solutions. In the rare-earth magnetobtained by the manufacturing method of the present invention, bycontrast, the protective layer has a first layer covering the magnetbody and containing a rare-earth element, and a second layer coveringthe first layer and containing a rare-earth element by an amount smallerthan that in the first layer. It seems that, since a surface of therare-earth magnet is covered with the second layer containing arare-earth element by an amount smaller than that in the first layer,the stability of the protective layer improves, thereby ameliorating thecorrosion resistance. It also seems that the protective layer having thespecific structure mentioned above becomes dense in structure, therebyimproving the stability of the protective layer and ameliorating thecorrosion resistance.

It will also be preferred in such a rare-earth magnet if the protectivelayer is formed by heat-treating the magnet body in an oxidizingatmosphere containing an oxidizing gas while adjusting at least onecondition of a partial pressure of the oxidizing gas, a treatmenttemperature, and a treatment time such as to have the first layercovering the magnet body and containing a rare-earth element, and thesecond layer covering the first layer and containing a rare-earthelement by an amount smaller than that in the first layer.

Preferably, in the rare-earth magnet of the present invention, theprotective layer contains oxygen and an element derived from the magnetbody. This makes the protective layer extremely excellent in adhesion tothe magnet body, thereby further improving the corrosion resistance ofthe rare-earth magnet. Such a rare-earth magnet of the present inventionhas a sufficiently high corrosion resistance, a uniform protective layerthickness, and an excellent dimensional precision. Also, since thespecific protective layer is formed, this rare-earth magnet is kept fromdeteriorating its performances at the time of manufacture and use, andhas an excellent reliability.

Specifically, it will be preferred if the magnet body contains arare-earth element and a transition element other than the rare-earthelement, the first layer contains the rare-earth element, the transitionelement, and oxygen, and the second layer contains the transitionelement and oxygen.

Namely, it will be preferred if the rare-earth element in the firstlayer, the transition element in the first layer, and the transitionelement in the second layer are elements derived from the magnet body.In particular, it will be more preferred if the rare-earth element inthe first layer, the transition element in the first layer, and thetransition element in the second layer are elements constructing a mainphase of the magnet body.

More preferably, in the protective layer, the rare-earth element isneodymium. Further, iron and/or cobalt is preferred as the transitionelement other than the rare-earth element.

More preferably, in the rare-earth magnet of the present invention, thefirst and second layers have a total thickness of 0.1 to 20 μm.

The rare-earth magnet of the present invention may comprise a magnetbody containing a rare-earth element, and a protective layer formed on asurface of the magnet body; the protective layer having an innerprotective layer containing a rare-earth element and/or a transitionelement and oxygen, and an outer protective layer made of a constituentmaterial different from that of the inner protective layer.

In recent years, the use of rare-earth magnets as magnets for motors inhybrid cars have been under consideration. In this case, the rare-earthmagnets are used near engines and are exposed to a high temperatureexceeding 150° C. However, conventional rare-earth magnets have beenlikely to deteriorate by corrosion in such a high-temperatureenvironment, and their protective layers have been insufficient in termsof heat resistance.

By contrast, the rare-earth magnet of the present invention comprisestwo protective layers, i.e., inner and outer protective layers havingrespective constituent materials different from each other, and thus isequipped with not only superior corrosion resistance but also superiorheat resistance, as compared with a conventional rare-earth magnetformed with a single protective layer.

More preferably, the inner protective layer in the rare-earth magnet ofthe present invention has a first layer covering the magnet body andcontaining a rare-earth element, and a second layer covering the firstlayer and containing substantially no rare-earth element.

In the inner protective layer having this structure, the first layeradjacent to the magnet body contains a rare-earth element and thusexhibits an excellent adhesion to the magnet body. The second layerformed on the outer side contains substantially no rare-earth element,and thus is very hard to oxidize. Therefore, the rare-earth magnetcomprising such first and second layers can exhibit a superior corrosionresistance as compared with one not provided with these two protectivelayers.

The inner protective layer may have a first layer covering the magnetbody and containing a rare-earth element, and a second layer coveringthe first layer and containing a rare-earth element by an amount smallerthan that in the first layer. Such a second layer is also very hard tooxidize. Therefore, the rare-earth magnet comprising such first andsecond layers can exhibit a superior corrosion resistance as comparedwith one not provided with these two protective layers.

More specifically, it will be preferred if the magnet body contains arare-earth element and a transition element other than the rare-earthelement, the first layer contains the rare-earth element, the transitionelement, and oxygen, and the second layer contains the transitionelement and oxygen. In this case, the first layer contains the samerare-earth element as with the magnet body, and the second layercontains the same transition element as with the first layer, wherebythe adhesion of each layer can become more favorable. As a result, thecorrosion resistance of the rare-earth magnet further improves.

In particular, it will be further preferred if the rare-earth element inthe first layer, the transition element in the first layer, and thetransition element in the second layer are elements derived from themagnet body. Namely, it will be preferred if the first and second layersare formed by changing the magnet body by a reaction or the like. Thisstructure makes the adhesion of each layer more favorable and allowseach layer to become a very dense film. As a result, the corrosionresistance of the rare-earth magnet becomes further favorable.

Preferably, in the rare-earth magnet of the present invention, the outerprotective layer is an oxide layer having a composition different fromthat of the inner protective layer. When an oxide layer having acomposition different from that of the inner protective layer is thusprovided on the outside of the inner protective layer, the rare-earthmagnet becomes extremely excellent in not only corrosion resistance butalso heat resistance. Such an effect becomes further superior inparticular when the oxide layer contains an oxide of a metal elementdifferent from a metal element contained in the first and second layers.

More preferably, the oxide layer is an amorphous layer. The outerprotective layer microscopically has no grain boundary. Usually, incrystalline substances, grain boundary parts deteriorate, therebycausing dropouts of particles and the like, which may become a cause ofcorrosion. When the oxide layer as the outer protective layer is madeamorphous as mentioned above, however, the corrosion can effectively berestrained from being caused as such.

More preferably, the oxide layer has a layer made of a p-type oxidesemiconductor, and a layer made of an n-type oxide semiconductor formedon the outer side thereof. It has been presumed that corrosion of arare-earth magnet occurs when a rare-earth element is oxidized, i.e.,the rare-earth element is deprived of an electron. Therefore, when alayer made of a p-type semiconductor oxide and a layer made of an n-typesemiconductor oxide are thus formed successively from the magnet bodyside, a rectifying action caused by such coupling inhibits electronsfrom flowing in the direction mentioned above. As a result, thecorrosion resistance of the rare-earth magnet further improves.

More preferably, it will be preferred if the outer protective layer isan oxide layer containing an oxide of at least one species of elementselected from the group consisting of Al, Ta, Zr, Hf, Nb, P, Si, Ti, Mg,Cr, Ni, Ba, Mo, V, W, Zn, Sr, Bi, B, Ca, Ga, Ge, La, Pb, In, and Mn. Thelayer made of oxides of these elements attains an excellent heatresistance. As the oxide layer, one containing an oxide of Mo or W ispreferred in particular.

A resin layer containing a resin is also preferred as the outerprotective layer. Providing the resin layer as the outer protectivelayer in addition to the inner protective layer can yield a rare-earthmagnet having an excellent heat resistance in addition to a sufficientcorrosion resistance.

As the resin contained in the resin layer as the outer protective layer,a thermosetting resin is preferred since it can exhibit a desirablecharacteristic even in a high-temperature environment (e.g., at 150° C.or higher).

It will be more preferred if the resin constructing the resin layer isat least one species of resin selected from the group consisting ofphenol, epoxy, and melamine resins in particular. These resins can formcured products having an extremely excellent heat resistance among resinmaterials. Therefore, the rare-earth magnet of the present inventionequipped with such an outer protective layer attains not only acorrosion resistance but also an extremely excellent heat resistance.

It will also be preferred if the outer protective layer in therare-earth magnet of the present invention is a metal salt layer. Such ametal salt layer can also enhance the heat resistance of the rare-earthmagnet. When a coating or the like is further provided on the surface ofthe rare-earth magnet, the metal salt layer can also exhibit acharacteristic of being able to enhance the adhesion between the magnetbody and the coating. Therefore, the rare-earth magnet of the presentinvention having the surface provided with the metal salt layer becomesexcellent in adhesion to coatings, and is also extremely excellent incorrosion resistance and heat resistance after coating.

It will be more preferred if the metal salt layer contains at least onespecies of element selected from the group consisting of Cr, Ce, Mo, W,Mn, Mg, Zn, Si, Zr, V, Ti, and Fe and at least one species of elementselected from the group consisting of P, O, C, and S. The metal saltlayer containing these elements attains extremely excellent corrosionresistance and heat resistance.

It will be more preferred if the metal salt layer contains at least onespecies of element selected from the group consisting of Mo, Ce, Mg, Zr,Mn, and W and at least one species of element selected from the groupconsisting of P, O, C, and S. The metal salt layer containing theseelements attain excellent corrosion resistance and heat resistance inparticular.

As the outer protective layer, a layer containing an organic-inorganichybrid compound having a structural unit made of an organic polymer anda structural unit made of an inorganic polymer is also preferred. Theouter protective layer containing such an organic-inorganic hybridcompound is also excellent in the effect of improving the heatresistance of the rare-earth magnet. Such an outer protective layer canexhibit not only the heat resistance but also the followingcharacteristics.

First, the structural unit made of an organic polymer has acharacteristic of being soft. Therefore, even when a volume changeoccurs in a layer containing such a structural unit because of heatingor the like applied thereto at the time of forming the layer, therebygenerating a stress or the like, the structural unit made of a softorganic polymer can sufficiently alleviate such a stress. Therefore, theouter protective layer is harder to form defects such as cracks andpinholes due to stresses generated at the time of forming. On the otherhand, a compound containing a structural unit made of an inorganicpolymer has not only an excellent heat resistance but also acharacteristic of being harder to transmit moisture and the liketherethrough (moisture permeation resistance).

The outer protective layer in the rare-earth magnet of the presentinvention contains an organic-inorganic hybrid compound having both ofthese structural units. Therefore, this outer protective layer has bothcharacteristics of these two structural units. Consequently, therare-earth magnet equipped with such an outer protective layer hasexcellent corrosion resistance, heat resistance, and moistureresistance.

However, studies by the inventors have revealed that, when a materialsimply mixing organic and inorganic molecules in order to obtain aprotective layer having both of the characteristics mentioned above, theorganic and inorganic molecules are easier to separate from each otherin the resulting protective layer, whereby there is a case where theprotective layer is formed with a region in which any of theabove-mentioned characteristics is insufficient.

By contrast, the outer protective layer in the present inventioncontains an organic-inorganic hybrid compound, i.e., a compound in whicha structural unit made of an organic polymer and a structural unit madeof an inorganic polymer are combined together by a predeterminedinteraction. Therefore, the two structural units are rarely separatedfrom each other in this layer. Consequently, the outer protective layerhaving this organic-inorganic hybrid compound has a homogenouscharacteristic throughout the layer, and can provide the rare-earthmagnet with excellent corrosion resistance, heat resistance, andmoisture resistance.

Specifically, it will be preferred if the organic-inorganic hybridcompound is a compound in which a structural unit made of an organicpolymer and a structural unit made of an inorganic polymer are combinedtogether by a covalent bond. Also preferred as the organic-inorganichybrid compound is a compound in which a structural unit made of anorganic polymer and a structural unit made of an inorganic polymer arecombined together by a hydrogen bond. The organic-inorganic hybridcompound may also be a compound in which a structural unit made of anorganic polymer having an aromatic ring and a structural unit made of aninorganic polymer having an aromatic ring are combined together by aninteraction between the aromatic rings.

Each of these organic-inorganic hybrid compounds is one in which astructural unit made of an organic molecule and a structural unit madeof an inorganic molecule are combined together by a predeterminedinteraction, and thus is less likely to cause separation and the like inthe outer protective layer. The rare-earth magnet equipped with an outerprotective layer containing such an organic-inorganic hybrid compound isextremely excellent in heat resistance and moisture resistance inaddition to corrosion resistance.

It will be more preferred in the rare-earth magnet of the presentinvention if the outer protective layer further contains an inorganicadditive. The outer protective layer further containing an inorganicadditive has a more heat resistance and is also excellent in terms ofstrength, whereby even shocks and the like exerted during themanufacture and use of the rare-earth magnet are less likely to causecracks and the like. Therefore, the rare-earth magnet equipped with suchan outer protective layer attains more excellent corrosion resistanceand heat resistance.

In another aspect, the present invention provides a method of favorablymanufacturing the rare-earth magnet of the present invention. Namely,the method of manufacturing a rare-earth magnet in accordance with thepresent invention is a method of manufacturing a rare-earth magnet byforming a protective layer on a surface of a magnet body containing arare-earth element, the method comprising a protective layer formingstep of heat-treating the magnet body so as to form a protective layerhaving a first layer covering the magnet body and containing arare-earth element and a second layer covering the first layer andcontaining substantially no rare-earth element.

The method of manufacturing a rare-earth magnet in accordance with thepresent invention may be a method of manufacturing a rare-earth magnetby forming a protective layer on a surface of a magnet body containing arare-earth element, the method comprising a protective layer formingstep of heat-treating the magnet body so as to form a protective layerhaving a first layer covering the magnet body and containing arare-earth element and a second layer covering the first layer andcontaining a rare-earth element by an amount smaller than that in thefirst layer.

Preferably, in the method of manufacturing a rare-earth magnet, themagnet body is heat-treated in the protective layer forming step in anoxidizing atmosphere containing an oxidizing gas while adjusting atleast one condition of a partial pressure of the oxidizing gas, atreatment temperature, and a treatment time such that the protectivelayer has the first layer and the second layer.

When at least one condition of the partial pressure of the oxidizinggas, treatment temperature, and treatment time at the time ofheat-treating the magnet body in an oxidizing atmosphere is adjustedwhile using the structure of a film (oxidized film) formed on thesurface of the rare-earth magnet as an index, corrosion can berestrained from occurring in excess in an oxidizing atmosphere in whichthe rare-earth magnet is likely to corrode, and a rare-earth magnethaving a sufficient corrosion resistance can be obtained. Such amanufacturing method can form a protective layer very easily at lowcost, a protective layer having a uniform thickness, and a rare-earthmagnet which is excellent in dimensional precision. In particular, itwill be preferred in this manufacturing method if the magnet body isheat-treated while adjusting the partial pressure of the oxidizing gas,treatment temperature, and treatment time. Adjusting these threeconditions can yield a rare-earth magnet having a sufficient corrosionresistance more easily and reliably.

Preferably, the manufacturing method of the present invention furthercomprises a pickling step of pickling the magnet body prior to the heattreatment. Pickling the magnet body prior to the above-mentioned heattreatment can remove denatured layers and oxidized layers formed on themagnet body surface during or after the manufacture of the magnet body,whereby a desirable protective layer can be formed more accurately.

Preferably, in the manufacturing method of the present invention, theoxidizing atmosphere is a steam atmosphere having a steam partialpressure of 10 to 2000 hPa. This allows the above-mentioned first andsecond layers to be formed favorably, whereby the corrosion resistanceof the rare-earth magnet further improves.

It will be more preferred in the manufacturing method of the presentinvention if the treatment time is 1 min to 24 hr. This allows theabove-mentioned first and second layers to be formed favorably, andmakes it very hard for the heat treatment and the like to deterioratecharacteristics of the magnet body.

The method of manufacturing a rare-earth magnet in accordance with thepresent invention may be a method of manufacturing a rare-earth magnetby forming a protective layer on a surface of a magnet body containing arare-earth element, the method comprising an inner protective layerforming step of heat-treating the magnet body so as to form an innerprotective layer covering the magnet body and containing a rare-earthelement and/or a transition element and oxygen, and an outer protectivelayer forming step of forming an outer protective layer made of aconstituent material different from that of the inner protective layeron a surface of the inner protective layer.

Such a manufacturing method can yield a rare-earth magnet comprising aplurality of protective layers, i.e., inner and outer protective layers,made of respective constituent materials different from each other,which is extremely excellent in heat resistance in addition to corrosionresistance.

It will be preferred in the inner protective layer forming step if themagnet body is heat-treated so as to form the inner protective layerhaving a first layer covering the magnet body and containing arare-earth element and a second layer covering the first layer andcontaining substantially no rare-earth element. The magnet body may beheat-treated so as to form the inner protective layer having a firstlayer covering the magnet body and containing a rare-earth element and asecond layer covering the first layer and containing a rare-earthelement by an amount smaller than that in the first layer. This formsthe first and second layers extremely excellent in corrosion resistanceas mentioned above as the inner protective layer, whereby the corrosionresistance of the resulting rare-earth magnet further improves.

Preferably, in the inner protective layer forming step, the magnet bodyis heat-treated in an oxidizing atmosphere containing an oxidizing gaswhile adjusting at least one condition of a partial pressure of theoxidizing gas, a treatment temperature, and a treatment time such thatthe protective layer has the first layer and the second layer. Adjustingthese conditions can favorably form the first and second layers.

Preferably, in the outer protective layer forming step, the outerprotective layer made of an oxide layer having a composition differentfrom the inner protective layer is formed on the surface of the innerprotective layer. The outer protective layer made of such an oxide layercan provide the rare-earth magnet with an excellent heat resistance.

In the outer protective layer forming step, a resin layer formingcoating liquid containing a resin may be applied onto the surface of theinner protective layer and dried so as to form the outer protectivelayer made of a resin layer. The rare-earth magnet equipped with thusformed resin layer is also extremely excellent in corrosion resistanceand heat resistance. When the resin is at least one species of resinselected from the group consisting of phenol, epoxy, and melamine resinsin particular, a more excellent heat resistance can be obtained.

In the outer protective layer forming step, the magnet body after theinner protective layer forming step may be subjected to chemicalconversion treatment so as to form the outer protective layer made of achemical conversion layer on the surface of the inner protective layer.Thus formed outer protective layer can also provide the rare-earthmagnet with an excellent heat resistance.

It will also be preferred in the outer protective layer forming step ifthe outer protective layer made of a layer containing anorganic-inorganic hybrid compound having a structural unit made of anorganic polymer and a structural unit made of an inorganic polymer isformed on the surface of the inner protective layer. The rare-earthmagnet equipped with the outer protective layer containing such anorganic-inorganic hybrid compound attains an excellent moistureresistance in addition to the corrosion resistance and heat resistanceas mentioned above.

Another method of manufacturing a rare-earth magnet in accordance withthe present invention is a method of manufacturing a rare-earth magnetby heat-treating a magnet body containing a rare-earth element so as toform a protective layer on a surface of the magnet body, the methodcomprising a pickling step of pickling the magnet body, and aheat-treating step of heat-treating the pickled magnet body in anoxidizing atmosphere containing an oxidizing gas. Such a heat-treatingstep is preferably performed subsequent to the pickling step, morepreferably immediately after the pickling.

Performing such a pickling step can remove a number of irregularities,oxidized layers, and processed and denatured layers on the magnet bodysurface, thereby cleaning the surface. This can form a desirableoxidized film more accurately in the heat-treating step after thepickling.

In particular, when the magnet body containing an unprocessed part ispickled in the pickling step after sintering, a rare-earth-rich layerwhich is likely to remain as oozing from within the magnet body to thesurface at the time of sintering can be removed. This is effective informing a desirable film in particular.

EFFECT OF THE INVENTION

The present invention can provide a rare-earth magnet having asufficient corrosion resistance, and a method of manufacturing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] A schematic perspective view showing the rare-earth magnet inaccordance with a first embodiment.

[FIG. 2] A view schematically showing a cross-sectional structureappearing when the rare-earth magnet shown in FIG. 1 is cut along theline II-II.

[FIG. 3] A schematic perspective view showing the rare-earth magnet inaccordance with a second embodiment.

[FIG. 4] A view schematically showing a cross-sectional structureappearing when the rare-earth magnet shown in FIG. 3 is cut along theline IV-IV.

[FIG. 5] An electron micrograph of the rare-earth magnet in accordancewith Example 1A.

[FIG. 6] An electron micrograph enlarging a part of FIG. 5.

[FIG. 7] An electron micrograph of the rare-earth magnet in accordancewith Comparative Example 1A.

[FIG. 8] An electron micrograph enlarging a part of FIG. 7.

[FIG. 9] An electron micrograph of the rare-earth magnet in accordancewith Example 1C.

[FIG. 10] An electron micrograph enlarging a part of FIG. 9.

[FIG. 11] An electron micrograph of the rare-earth magnet in accordancewith Comparative Example 1C.

[FIG. 12] An electron micrograph enlarging a part of FIG. 11.

[FIG. 13] An electron micrograph of the rare-earth magnet in accordancewith Example 2C prior to a salt spray test.

[FIG. 14] An electron micrograph of the rare-earth magnet in accordancewith Example 2C at 24 hr after starting the salt spray test.

[FIG. 15] An electron micrograph of the rare-earth magnet in accordancewith Comparative Example 1C prior to the salt spray test.

[FIG. 16] An electron micrograph of the rare-earth magnet in accordancewith Comparative Example 1C at 24 hr after starting the salt spray test.

[FIG. 17] An electron micrograph of the rare-earth magnet in accordancewith Reference Example 1C prior to the salt spray test.

[FIG. 18] An electron micrograph of the rare-earth magnet in accordancewith Reference Example 1C at 24 hr after starting the salt spray test.

EXPLANATIONS OF NUMERALS

1 . . . rare-earth magnet; 3 . . . magnet body; 5 . . . protectivelayer; 5 a . . . first layer; 5 b . . . second layer; 10 . . .rare-earth magnet; 13 . . . magnet body; 15 . . . protective layer; 16 .. . first layer; 17 . . . second layer; 18 . . . inner protective layer;19 . . . outer protective layer.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, preferred embodiments of the present invention will beexplained in detail with reference to the drawings as necessary. In thedrawings, the same constituents will be referred to with the samenumerals without repeating their overlapping explanations. Positionalrelationships such as upper, lower, left, and right are based on thoseshown in the drawings unless otherwise specified. Dimensional ratios inthe drawings are not limited to those depicted.

First Embodiment

To begin with, a first embodiment of the rare-earth magnet and method ofmanufacturing the same in accordance with the present invention will beexplained. The rare-earth magnet of the first embodiment comprises amagnet body containing a rare-earth element, and a protective layerformed on a surface of the magnet body, whereas the protective layerincludes a first layer covering the magnet body and containing arare-earth element, and a second layer covering the first layer andcontaining substantially no rare-earth element.

FIG. 1 is a schematic perspective view showing the rare-earth magnet inaccordance with the first embodiment. FIG. 2 is a view schematicallyshowing a cross-sectional structure appearing when the rare-earth magnetshown in FIG. 1 is cut along the line II-II. As shown in FIGS. 1 and 2,the rare-earth magnet 1 of this embodiment is constructed by a magnetbody 3 and a protective layer 5 formed so as to cover all the surfacesof the magnet body 3.

Magnet Body

The magnet body 3 is a permanent magnet containing a rare-earth element.In this case, the rare-earth element refers to scandium (Sc), yttrium(Y), and lanthanide elements belonging to Group 3 in the long-periodperiodic table. Examples of the lanthanide elements include lanthanum(La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

Examples of constituent materials for the magnet body 3 include thosecontaining the rare-earth element and a transition element other thanthe rare-earth element in combination. In this case, the rare-earthelement is preferably at least one species of element selected from thegroup consisting of Nd, Sm, Dy, Pr, Ho, and Tb, and more preferablyfurther contains at least one species of element selected from the groupconsisting of La, Ce, Gd, Er, Eu, Tm, Yb, and Y in addition to theformer elements.

Preferred as the transition element other than the rare-earth element isat least one species of element selected from the group consisting ofiron (Fe), cobalt (Co), titanium (Ti), vanadium (V), chromium (Cr),manganese (Mn), nickel (Ni), copper (Cu), zirconium (Zr), niobium (Nb),molybdenum (Mo), hafnium (Hf), tantalum (Ta), and tungsten (W), morepreferred being Fe and/or Co.

More specific examples of constituent materials for the magnet body 3include those based on R—Fe—B and R—Co. Rare-earth elements mainlycomposed of Nd are preferred as R in the former constituent material,whereas rare-earth elements mainly composed of Sm are preferred as R inthe latter constituent material.

Preferred in particular as constituent materials for the magnet body 3are those based on R—Fe—B. Such a material has a main phase of asubstantially tetragonal crystal structure, whereas a rare-earth-richphase with a higher compounding ratio of a rare-earth element and aboron-rich phase with a higher compounding ratio of boron atoms areprovided near a grain boundary part of the main phase. Therare-earth-rich phase and boron-rich phase are nonmagnetic phaseswithout magnetism. A magnet constituent material usually contains suchnonmagnetic phases by 0.5 to 50 vol %. The particle size of the mainphase is usually about 1 to 100 μm.

It will be preferred in such an R—Fe—B-based constituent material if therare-earth element content is 8 to 40 atom %. When the rare-earthelement content is less than 8 atom %, the main phase attainssubstantially the same crystal structure as that of α-iron, wherebycoercive force (iHc) tends to decrease. When the content exceeds 40 atom%, on the other hand, the rare-earth-rich phase is formed in excess,whereby residual magnetic flux density (Br) tends to decrease.

Preferably, the Fe content is 42 to 90 atom %. The residual magneticflux density tends to decrease when the Fe content is less than 42 atom%, whereas the coercive force tends to decrease when the content exceeds90 atom %. Preferably, the B content is 2 to 28 atom %. When the Bcontent is less than 2 atom %, a rhombohedral structure is likely toform, whereby the coercive force tends to decrease. When the B contentexceeds 28 atom %, the boron-rich phase is formed in excess, whereby theresidual magnetic flux density tends to decrease.

In the above-mentioned constituent material, Fe in the R—Fe—B system maypartly be replaced by Co. Thus partly replacing Fe with Co can improvethe temperature characteristic without lowering the magneticcharacteristic. In this case, it will be desirable if the amountreplaced by Co is not greater than the Fe content. When the Co contentexceeds the Fe content, the magnetic characteristic of the magnet bodytends to decrease.

B in the constituent material may partly be replaced by an element suchas carbon (C), phosphorus (P), sulfur (S), or copper (Cu). Thus partlyreplacing B makes it easier to manufacture the magnet body and can cutdown the manufacturing cost. Here, the amount replaced by these elementsis desirably an amount which does not substantially affect the magneticcharacteristic, and is preferably 4 atom % or less with respect to thetotal amount of constituent atoms.

From the viewpoint of improving the coercive force, cutting down themanufacturing cost, and so forth, elements such as aluminum (Al),titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), bismuth(Bi), niobium (Nb), tantalum (Ta), molybdenum (Mo), tungsten (W),antimony (Sb), germanium (Ge), tin (Sn), zirconium (Zr), nickel (Ni),silicon (Si), gallium (Ga), copper (Cu), and hafnium (Hf) may be addedto the above-mentioned structure. Their added amount preferably fallswithin a range not affecting the magnetic characteristic, and is 10 atom% or less with respect to the total amount of constituent atoms. Inaddition, oxygen (O), nitrogen (N), carbon (C), calcium (Ca), and thelike are considered to be inevitably mingling components. They may becontained by an amount of about 3 atom % or less with respect to thetotal amount of constituent atoms.

The magnet body 3 having such a structure can be manufactured by powdermetallurgy. First, in this method, an alloy having a desirablecomposition is made by a known alloy manufacturing process such ascasting or strip casting. Subsequently, the alloy is pulverized into aparticle size of 10 to 100 μm with a coarse pulverizer such as jawcrusher, Brown mill, or stamp mill, and then further into a particlesize of 0.5 to 5 μm with a fine pulverizer such as jet mill or attritor.Thus obtained powder is molded at a pressure of 0.5 to 5 t/cm²preferably in a magnetic field having a magnetic field intensity of 600kVA/m or greater.

Thereafter, thus obtained molded body is sintered for 0.5 to 10 hr at1000 to 1200° C. preferably in an inert gas atmosphere or vacuum, andthen is rapidly cooled. This sintered body is further heat-treated for 1to 5 hr at 500 to 900° C. in an inert gas atmosphere or vacuum, and isprocessed into a desirable form (practical form) as necessary, so as toyield the magnet body 3.

Preferably, thus obtained magnet body 3 is further pickled. Namely, itwill be preferred if the surface of the magnet body 3 is pickled priorto heat treatment which will be explained later.

Nitric acid is preferable as an acid used in pickling. Nonoxidizingacids such as hydrochloric acid and sulfuric acid are often used whenplating typical steel materials. When the magnet body 3 containing arare-earth element as in this embodiment is treated with these acids,however, hydrogen generated by the acids is occluded in the surface ofthe magnet body 3, so that the occluded part embrittles, therebygenerating a large amount of powdery undissolved product. Since thepowdery undissolved product causes roughening, defects, poor adhesion,and the like after surface treatment, it will be preferred if theabove-mentioned nonoxidizing acids are not contained in a picklingtreatment liquid. Therefore, nitric acid which is an oxidizing acidgenerating less hydrogen is preferably used.

The amount of the surface of the magnet body 3 dissolved by suchpickling is preferably 5 μm or greater, more preferably 10 to 15 μm, byan average thickness from the surface. Completely removing denaturedlayers and oxidized layers due to processing of the surface of themagnet body 3 allows heat treatment, which will be explained later, toform a desirable oxidized film more accurately.

The nitric acid concentration of a processing liquid used for picklingis preferably 1 N or less, 0.5 N or less in particular. When the nitricacid concentration is too high, the dissolving rate of the magnet body 3becomes so high that the dissolving amount is harder to regulate, whichcauses a greater deviation in bulk processing such as barrel processingin particular, whereby the dimensional precision of the product isharder to maintain. When the nitric acid concentration is too low, thedissolving amount tends to be insufficient. Therefore, the nitric acidconcentration is preferably 1 N or less, 0.5 to 0.05 N in particular.The dissolving amount of Fe at the end of processing is about 1 to 10g/l.

It will be preferred if the pickled magnet body 3 is subjected towashing with ultrasonic waves in order to completely remove a smallamount of undissolved products and remaining acid components. It will bepreferred if the ultrasonic washing is performed in deionized water inwhich the amount of chlorine ions generating rust on the surface of themagnet body is very small. If necessary, similar water washing may beperformed before and after the ultrasonic washing and in each step ofpickling.

Protective Layer

The protective layer 5 contains an element derived from the magnet body3 and oxygen, and has a first layer 5 a covering the magnet body 3 andcontaining a rare-earth element, and a second layer 5 b covering thefirst layer 5 a and containing a rare-earth element by an amount smallerthan that in the first layer. More specifically, it contains an elementconstructing the above-mentioned main phase in the magnet body 3 andoxygen.

Here, the element derived from the magnet body 3 is a constituentmaterial of the magnet body 3, contains at least a rare-earth elementand a transition element other than the rare-earth element, and mayfurther contain B, Bi, Si, Al, and the like. The protective layer 5 isneither applied nor attached onto the magnet body 3, but is constitutedby an element appearing on the magnet body 3 as the magnet body 3 itselfchanges by oxidizing and the like. Therefore, the protective layer 5does not contain new metal elements which do not construct the magnetbody, but may contain nonmetal elements such as oxygen and nitrogen.

The first layer 5 a contains an element derived from the magnet body 3such as a rare-earth element, and oxygen, and more specifically containsoxygen, a rare-earth element, and a transition element other than therare-earth element. When the constituent material for the magnet body 3is based on R—Fe—B, the transition element is mainly composed of Fe andmay contain Co and the like depending on the composition of theconstituent material.

While the second layer 5 b contains an element derived from the magnetbody 3 and oxygen, its rare-earth element content is smaller than thatin the first layer. When the constituent material for the magnet body 3is based on R—Fe—B, the transition element is mainly composed of Fe andmay contain Co and the like depending on the composition of theconstituent material. From the viewpoint of attaining more excellentcorrosion resistance due to the second layer 7, the second layer 7preferably contains a rare-earth element by an amount which is one halfor less of that in the first layer 6, and more preferably is a layercontaining substantially no rare-earth element. Namely, it will bepreferred in particular if the second layer 7 contains oxygen and atransition element other than the rare-earth element contained in themagnet body 3.

Contents of individual constituent materials in the first layer 5 a andsecond layer 5 b can be verified by a known composition analyzing methodsuch as EPMA (x-ray microanalyzer method), XPS (x-ray photoelectronspectroscopy), AES (Auger electron spectroscopy), or EDS(energy-dispersive fluorescent x-ray spectroscopy).

Here, a mode in which no rare-earth element is detected by theabove-mentioned EPMA, XPS, AES, or EDS is considered to be a modecontaining substantially no rare-earth element. Namely, in the secondlayer 5 b, the rare-earth element content is about the detection limitby the above-mentioned composition analyzing method or less. In otherwords, the second layer 5 b may contain a rare-earth element by anamount not exceeding the detection limit by the above-mentionedcomposition analyzing method.

The protective layer 5 is formed by heat-treating (heating) the magnetbody 3 in an oxidizing atmosphere containing an oxidizing gas whileadjusting at least one of a partial pressure of the oxidizing gas, atreatment temperature, and a treatment time such that the protectivelayer attains the above-mentioned structure. It will be preferred ifthree conditions of the oxidizing gas, treatment temperature, andtreatment time are adjusted at the time of heat treatment.

Here, the oxidizing atmosphere is not limited in particular as long asit is an atmosphere containing an oxidizing gas, examples of whichinclude atmospheres promoting oxidization such as air, oxygenatmospheres (preferably atmospheres with a regulated oxygen partialpressure) and steam atmospheres (preferably atmospheres with a regulatedsteam partial pressure). Though not restricted in particular, examplesof the oxidizing gas include oxygen and steam. For example, an oxygenatmosphere is an atmosphere whose oxygen concentration is at least 0.1%,whereas an inert gas coexists with oxygen. Namely, a mode of the oxygenatmosphere is an atmosphere composed of oxygen and an inert gas. Forexample, the steam atmosphere is an atmosphere having a steam partialpressure of 10 hPa or higher, whereas an inert gas coexists with steamin this atmosphere. An example of the inert gas is nitrogen, whereas amode of the steam atmosphere is an atmosphere composed of steam and theinert gas. The oxidizing gas is preferably a steam atmosphere, since theprotective layer can be formed more easily thereby. Another example ofthe oxidizing atmosphere is an atmosphere containing oxygen, steam, andan inert gas.

First, when adjusting the condition mentioned above, a correlationbetween the structure of the protective layer 5 and at least onecondition of the oxidizing gas partial pressure, treatment temperature,and treatment time is determined. Next, according to this correlation,at least one condition of the oxidizing gas partial pressure, treatmenttemperature, and treatment time is adjusted at the time of heattreatment such that the protective layer 5 attains the specificstructure mentioned above.

Here, the treatment temperature is preferably adjusted within the rangeof 200 to 550° C., more preferably within the range of 250 to 500° C.The magnetic characteristic tends to deteriorate when the treatmenttemperature exceeds the upper limit mentioned above, whereas a desirableoxidized film is harder to form when the temperature is less than thelower limit mentioned above.

The treatment time is preferably adjusted within the range of 1 min to24 hr, more preferably within the range of 5 min to 10 hr. The magneticcharacteristic tends to deteriorate when the treatment time exceeds theupper limit mentioned above, whereas a desirable oxidized film is harderto form when the time is less than the lower limit mentioned above.

When the oxidizing atmosphere is a steam atmosphere here, a correlationbetween the structure of the protective layer 5 and at least onecondition of the steam partial pressure, treatment temperature, andtreatment time is determined at first. Next, according to thiscorrelation, at least one condition of the steam gas partial pressure,treatment temperature, and treatment time is adjusted at the time ofheat treatment such that the protective layer 5 attains the specificstructure mentioned above.

In this case, it will be preferred if the treatment temperature andtreatment time are adjusted within the ranges mentioned above. The steampartial pressure is preferably adjusted within the range of 10 to 2000hPa. When the steam partial pressure is less than 10 hPa, it is harderfor the protective layer 5 to attain the two-layer structure mentionedabove. When the partial pressure exceeds 2000 hPa, on the other hand,this high pressure not only complicates the system structure, but alsomakes condensation and the like easier to occur, etc., wherebyworkability tends to become worse.

The total thickness of the first layer 5 a and second layer 5 b ispreferably greater than 0.1 μm, more preferably 1 μm or greater. Whenthe total thickness is 0.1 μm or less, a protective layer having atwo-layer structure is harder to form. On the other hand, the totalthickness of the first layer 5 a and second layer 5 b is preferably lessthan 20 μm, more preferably 5 μm or less. When the total thickness is 20μm or greater, the oxidized film is harder to form, or the magneticcharacteristic tends to become lower.

The thickness of the second layer 5 b is preferably 5 nm or greater.When the thickness is less than 5 nm, it is too thin, whereby the effectof suppressing corrosion tends to become insufficient.

Second Embodiment

A second embodiment of the rare-earth magnet and method of manufacturingthe same in accordance with the present invention will now be explained.The rare-earth magnet of the second embodiment comprises a magnet bodycontaining a rare-earth element, and a protective layer formed on asurface of the magnet body, whereas the protective layer includes aninner protective layer containing a rare-earth element and/or atransition element and oxygen, and an outer protective layer made of aconstituent material different from that of the inner protective layer.In the rare-earth magnet of the second embodiment, the inner protectivelayer has a structure comprising a first layer covering the magnet bodyand containing a rare-earth element, and a second layer covering thefirst layer and containing substantially no rare-earth element.

FIG. 3 is a schematic perspective view showing the rare-earth magnet inaccordance with the second embodiment. FIG. 4 is a view schematicallyshowing a cross-sectional structure appearing when the rare-earth magnetshown in FIG. 3 is cut along the line IV-IV. As shown in FIGS. 3 and 4,the rare-earth magnet 10 of this embodiment is constructed by a magnetbody 13 and a protective layer 15 formed so as to cover all the surfacesof the magnet body 3. The protective layer 15 has an inner protectivelayer 18 and an outer protective layer 19 successively from the magnetbody 13 side. The inner protective layer 18 comprises a first layer 16and a second layer 17 successively from the magnet body 13 side.Individual constituents of the rare-earth magnet 10 will now beexplained.

Magnet Body

The magnet body 13 is a permanent magnet containing a rare-earthelement, and preferably contains a rare-earth element and a transitionelement other than this rare-earth element. Preferred as such a magnetbody 13 is one having the same structure as that shown in theabove-mentioned first embodiment.

Protective Layer

The protective layer 15 comprises the inner protective layer 18 andouter protective layer 19 successively from the magnet body 13 side asmentioned above. The inner protective layer 18 comprises the first layer16 and second layer 17 successively from the magnet body 13 side,whereas examples of such first layer 16 and second layer 17 includethose similar to the first layer 5 a and second layer 5 b in theabove-mentioned first embodiment.

The outer protective layer 19 is a layer formed on the surface of theinner protective layer 18 and, unlike the inner protective layer 18, isnot a layer formed by a reaction of the magnet body 13, but a layernewly and separately formed on the surface of the magnet body 13.Therefore, the outer protective layer 19 does not contain elementsderived from the magnet body 13.

While the outer protective layer 19 may be made of various constituentmaterials, any of an oxide layer, a resin layer, a metal salt layer, anda layer containing an organic-inorganic hybrid compound is preferred inthis embodiment. Each of these outer protective layers 19 will now beexplained.

(1) Oxide Layer

The oxide layer is formed so as to cover the inner protective layer 18(second layer 17), and is a layer made of an oxide having a compositiondifferent from the inner protective layer 18.

Such an oxide layer may be either crystalline or amorphous, but ispreferably amorphous. An amorphous oxide layer has less grain boundaryparts which are relatively easier to deteriorate in a crystallinestructure, and thus can exhibit excellent corrosion resistance and heatresistance.

Examples of the oxide layer include layers made of metal oxides. A layerconstructed by an oxide of Al, Ta, Zr, Hf, Nb, P, Si, Ti, Mg, Cr, Ni,Ba, Mo, V, W, Zn, Sr, Bi, B, Ca, Ga, Ge, La, Pb, In, or Mn, for example,is preferred, whereas a layer may contain a plurality of species ofthem. Among them, an oxide of Mo, Mg, or W is preferred, an oxide of Moor W is more preferred, and an oxide of Mo is preferred in particular.These oxide layers can exhibit particularly excellent corrosionresistance and heat resistance. Though a preferred oxide layer containsan oxide of any of the above-mentioned elements, it is not alwaysconstructed by such an oxide alone, but oxygen in the oxide may bepartly replaced by nitrogen (N), sulfur (S), or the like. A specificexample is silicon oxynitride (SiO_(x)N_(1-x) (0<x<1)). In general,SiO_(x)N_(1-x) (0<x<1) is an n-type semiconductor.

From the viewpoint of attaining a better corrosion resistance, it willbe preferred in the protective layer 15 equipped with an oxide layer asthe outer protective layer 19 if the outer protective layer 19 comprisesa layer made of a p-type oxide semiconductor and a layer made of ann-type oxide semiconductor formed on the outer side thereof. The secondlayer 17 may be constructed by a p-type oxide semiconductor, while theoxide layer may be constructed by an n-type oxide semiconductor. Such astructure makes it harder for the rare-earth element contained in themagnet body 13 to cause an oxidizing reaction, whereby the deteriorationof not only the magnet body 13 but also the rare-earth magnet 10 iseffectively reduced.

Examples of combinations of such an outer protective layer 19 includecombinations with an oxide layer formed from an oxide of Cr, Cu, Mn, orNi when the magnet body 13 is made of a constituent material based onR—Fe—B.

Examples of methods of forming the outer protective layer 19 made of anoxide layer include known film-forming techniques typified byvapor-phase growth methods such as vacuum deposition, sputtering, ionplating, CVD, and thermal spraying; liquid-phase growth methods such ascoating and solution deposition; and sol-gel method. Among them, avapor-phase growth method (dry process) is preferably used, and reactivevapor deposition, reactive sputtering, reactive ion plating, plasma CVD,thermal CVD, or Cat-CVD is more preferably used. Such a dry process canprevent functions of the rare-earth magnet 10 from lowering as theconstituent material of the magnet body 13 is eluted.

From the viewpoint of forming the oxide layer at lower cost, a methodwhich can uniformly form a large area at once is preferred. Examples ofsuch a method of forming an oxide layer include sputtering and CVD.Their specific methods may adopt and employ a film-forming technique ofuniformly forming a layer having a large area, which has beenestablished in the field of flat panel displays and the like.

For example, when forming an oxide layer made of an oxide semiconductoras mentioned above, atmospheric thermal CVD using alkoxide as a materialis preferably employed. This method can inexpensively form an oxidelayer having a favorable quality. Examples of the alkoxide used in thematerial include metal alkoxides such as Si(OC₂H₅)₄, B(OCH₃)₃,B(OC₂H₅)₃, Ge(OC₂H₅)₄, Al(CH₃COCHCOCH₃)₂, Al(O-i-C₃H₇)₃, Ga(O-i-C₃H₇)₃,In(O-i-C₃H₇)₃, Sn(O-i-C₃H₇)₄, Pb(O-i-C₃H₇)₂, Bi(O-t-C₅H, 1)₃,Ti(O-i-C₃H₇)₄, TiO(CH₃COCHCOCH₃)₂, V(OCH₂H₅)₃, VO(CH₃COCHCOCH₃)₂,Cr(CH₃COCHCOCH₃)₃, Fe(O-i-C₃H₇)₃, Co(CH₃COCHCOCH₃)₃, Co(CH₃COCHCOCH₃)₂,Ni(O₂C₅H₇)₃, Ni(CH₃COCHCOCH₃)₂, Cu(O₂C₅H₇)₃, Cu(CH₃COCHCOCH₃)₂,Zn(OC₂H₅)₂, Zn(CH₃COCHCOCH₃)₂, Zr(O-i-C₃H₇)₄, Zr(O-t-C₄H₉)₄,Zr(O-n-C₄H₉)₄, Nb(OC₂H₅)₅, Mo(OC₂H₅)₅, Hf(O-i-C₃H₇)₄, Ta(OC₂H₅)₅,W(OC₂H₅)₅, Mg(OC₂H₅)₂, Ca(OC₂H₅)₂, Sr(O-i-C₃H₇)₂, Ba(OC₂H₅)₂,La(O-i-C₃H₇)₃, P(OCH₃)₃, PO(OCH₃)₃, PO(OC₂H₅)₃, Cr(OC₂H₅)₃, Mo(C₅H₅O₂)₂,Mo(C₅H₇O₂)₂, and MoO₂(C₅H₇O₂)₂.

In general, vacuum deposition employs a point source as its vapordeposition source, and thus is disadvantageous when used for forming adisplay which requires a layer having a large area to be formeduniformly at once. However, the rare-earth magnet 10 of this embodimentis relatively small in size, whereby the oxide layer can easily beformed by vapor deposition as well. Nevertheless, the cost of formingthe oxide layer tends to be higher in vapor deposition, since the areaof a film formed thereby at once is small. Therefore, when using vapordeposition, it is desirable that the film-forming rate be raised inorder to lower the cost of forming the oxide layer. When thefilm-forming rate is too high, however, coarse particles such assplashes may occur, thereby failing to yield the oxide layer having auniform surface.

Ion plating is a technique in which a coating material (a coatingmaterial for the oxide layer in this embodiment) and a substrate to becoated (the magnet body 13 formed with the inner protective layer 18 inthis embodiment) are arranged as an anode and a cathode in apressure-reduced container, the anode is heated in the presence orabsence of a reactive gas, so as to turn the coating material intoatoms, molecules, or fine particles, which are then ionized bythermoelectrons or the like and attached to the substrate to be coatedat the cathode.

Employable as a method of heating a material to be ionized in the ionplating is resistance heating of crucible type or direct resistanceheating type, high-frequency induction heating, or electron-beamheating. Among them, resistance heating tends to be unsuitable forforming an inorganic compound having a low vapor pressure. Thoughelectron beam heating can evaporate various materials, coarse particlessuch as splashes may occur when the film-forming rate is high, wherebythe oxide layer having a uniform surface may not be obtained.

Ion plating employs a point vapor deposition source, and thus tends tobe harder to form an oxide layer at relatively low cost as with theabove-mentioned vacuum deposition. For forming an oxide layer atrelatively low cost by using ion plating, it will be sufficient if afilm-forming apparatus utilizing a high-density plasma by apressure-gradient hollow cathode type plasma gun proposed in “MonthlyDISPLAY”, September 1999 issue, p. 28 is employed. This method is onekind of ion plating, and uses a sheet-like plasma described in JapanesePatent Application Laid-Open No. HEI 2-209475, thereby being able toform a layer having a large area uniformly at relatively low cost. Also,the ionizing ratio of the plasma gun is much higher in this method thanthat conventionally available, so that evaporated particles attain ahigher ratio of ionization, which tends to be effective in that the filmdensity can be kept high even when the substrate temperature isrelatively low, an effect of improving film qualities such ascrystallinity including the surface form and reactivity is obtained, andso forth.

Though not restricted in particular, the film-forming temperature at thetime of forming the oxide layer is preferably such that the heat historyat the time of film-forming does not deteriorate the magneticcharacteristic of the magnet body 13. From such a viewpoint, thefilm-forming temperature is preferably 500° C. or less, more preferably300° C. or less.

Though the composition of the atmosphere gas at the time of forming theoxide layer is not limited in particular, it will be preferred if thefilm-forming rate, the substrate temperature, or the oxygenconcentration in the atmosphere gas is adjusted, for example, when theoxygen content in the oxide layer is to be made smaller than thestoichiometric amount of oxygen in an oxide constituting the oxidelayer. Specifically, when the film-forming condition is adjusted suchthat the film-forming rate becomes 0.4 nm/sec or higher in the casewhere aluminum oxide is used as a constituent material for the oxidelayer, for example, the oxygen content in the resulting oxide layertends to become less than 1.5 times the Al content based on atoms. Here,the film-forming condition refers to a condition under which a substanceto be ionized is heated in the case of the above-mentioned ion plating,for example. Also, the input power in resistance heating andhigh-frequency induction heating, the amount of current of electronbeams in electron-beam heating, and so forth correspond to theirfilm-forming conditions.

When forming the oxide layer, a metal element constituting an oxide mayinitially be formed, and then postprocessing such as high-temperatureoxidization, plasma oxidization, or anode oxidization may be performed,so as to regulate the amount of oxygen.

Another example of the method of forming the oxide layer is diffusioncoating. The diffusion coating is a method in which a film of a metal orthe like is formed by sputtering or the like, and then is heated to 200to 500° C., so as to be oxidized in the air.

Though the above-mentioned example exemplifies a single-layer structureas the oxide layer acting as the outer protective layer 19, the oxidelayer may be constructed by a plurality of layers. Though the oxidelayer is supposed to contain no element derived from the magnet body, itmay contain an element derived from the magnet body by moving throughthe inner protective layer 18, for example, to such an extent thatcharacteristics of the layer are not lowered thereby.

(2) Resin Layer

A resin layer, which is another example of the outer protective layer19, is formed so as to cover the inner protective layer 18 (second layer17), and is constructed so as to contain a resin. Though the resin maybe either a synthetic resin or a natural resin, a synthetic resin ispreferred, and a thermosetting resin is more preferred.

Examples of the thermosetting resin include phenol resins, epoxy resins,urethane resins, silicone resins, melamine resins, epoxy-melamineresins, and thermosetting acrylic resins. Examples of thermoplasticresins include vinyl resins made from vinyl compounds such as acrylicacid, ethylene, styrene, vinyl chloride, and vinyl acetate. The resinlayer may also contain metal particles, oxide particles, and the like.

The resin layer is formed by using the resins mentioned above. Namely,it can be formed by dissolving any of the above-mentioned resins into anorganic solvent, so as to prepare a coating liquid for forming a resinlayer, applying the coating liquid onto the surface of the innerprotective layer 18, and then drying the coating liquid.

Though the coating method at the time of forming the resin layer is notlimited in particular, its examples include dip coating, dip-spincoating, and spray coating. The resin layer may be formed by applyingthe coating liquid for forming a resin layer once or a plurality oftimes. Forming the resin layer by applying the coating liquid aplurality of times is less likely to yield uncoated parts.

The thickness of the resin layer acting as the outer protective layer 19is preferably 0.1 to 100 μm, more preferably 1 to 50 μm.

The resin constituting the resin layer acting as the outer protectivelayer 19 is preferably a layer containing a phenol resin, epoxy resin,or melamine resin among those mentioned above. More preferred is a layercontaining a phenol resin or epoxy resin and a melamine resin incombination in particular.

Examples of phenol resins include alkylphenol resins and alkylpolyhydric phenol resins, and are exemplified by those obtained bycuring monomers or oligomers of alkyl phenols and alkyl polyhydricphenols or their mixtures. Curing can be effected, for example, bycausing the above-mentioned monomers and the like to react withformaldehyde, so as to yield a resol, and polymerizing thus obtainedresol, or by a method causing urushiol to react with water.

An example of the alkylphenol or alkyl polyhydric phenol is the compoundrepresented by the following general formula (1):

In the formula, R¹¹ and R¹² each indicate a hydroxyl or alkyl group,whereas at least one of R¹¹ and R¹² is an alkyl group. Preferred inparticular is an alkyl polyhydric phenol having, with respect to thehydroxy group in the formula, a hydroxyl group at an ortho position andan alkyl group at a meta or para position.

Preferred as such an alkyl polyhydric phenol in general is a componentcontained in a lacquer paint, specific examples thereof include urushiolhaving —CH₁₇H₂₅ at the meta position, thitsiol having —CH₁₇H₃₃ at thepara position, and laccol having —CH₁₇H₃₁ at the meta position.

The above-mentioned alkylphenol or alkyl polyhydric phenol can act as areducing agent, so that the magnet body 13 is covered with a strongreducing atmosphere even when heat-treated at a high temperature forcuring at the time of forming the outer protective layer 19 made of thisresin, whereby the deterioration due to oxidation of the magnet body 13can be decreased greatly.

Though the epoxy resin is not restricted in particular, employableexamples thereof include epoxy compounds such as those of bisphenoltype, polyol glycidyl ether type, polyacid glycidyl ester type,polyamine glycidylamine type, and alicyclic epoxy type. Preferably, inaddition to the above-mentioned epoxy compound, the epoxy resin furthercontains a curing agent which can cure the epoxy compound. Examples ofthe curing agent include polyamines, epoxy-resin-added products ofpolyamines, polyamide amines, and polyamide resins, whereas specificexamples include m-xylenediamine, isophoronediamine, diethylenetriamine,triethylenetetramine, and diaminodiphenylmethane.

The melamine resin is a resin formed by causingmelamine(2,4,6-triamino-1,3,5-triazine) to react with formaldehyde, soas to yield methylolamine, and then curing it. Though such a melamineresin may form the outer protective layer 19 by itself, it will be morepreferred if the melamine resin is used in combination with theabove-mentioned phenol resin and epoxy resin.

Since the melamine resin can form many crosslinking structures in thephenol resin or epoxy resin, the outer protective layer 19 containingthem in combination is extremely excellent in heat resistance andstrength. As a result, the corrosion resistance and heat resistance ofthe rare-earth magnet 10 further improve.

The outer protective layer containing the phenol resin, epoxy resin, ormelamine resin can be formed, for example, by dissolving or dispersingsuch a resin into a solvent, so as to yield a solution or varnish,applying it onto the surface of the inner protective layer 18, drying itand so forth as appropriate, and then curing the resin by heating or thelike.

(3) Metal Salt Layer

The metal salt layer acting as the outer protective layer 19 is formedso as to cover the inner protective layer 18 (second layer 17), and ismainly constructed by a metal salt.

It will be preferred in particular if the outer protective layer 19constructed by the metal salt is a chemical conversion layer formed bychemical conversion treatment of the magnet body 13 formed with theinner protective layer 18. Such a chemical conversion layer has a formin which a number of small planar crystals made of the metal salt areattached to the inner protective layer 18 (second layer 17) so as tocover it, for example.

Examples of the metal salt constructing the metal salt layer (chemicalconversion layer) include those containing at least one species of metalelement selected from the group consisting of Cr, Ce, Mo, W, Mn, Mg, Zn,Si, Zr, V, Ti, and Fe, whereas preferred are those containing theseelements and at least one species of element selected from the groupconsisting of P, O, C, and S. Specifically, phosphates or sulfates ofthe above-mentioned metal elements are preferred, and the phosphates aremore preferred.

In particular, those containing at least one species of metal elementselected from the group consisting of Mo, Ce, Mg, Zr, Mn, and W and atleast one species of element selected from the group consisting of P, O,C, and S are preferred, phosphates or sulfates of the above-mentionedmetal elements are more preferred, and the phosphates are preferred inparticular.

As mentioned above, the metal salt layer (chemical conversion layer) canfavorably be formed by chemical conversion treatment of the surface ofthe magnet body 13 formed with the inner protective layer 18. First, forchemical conversion treatment, the surface of the magnet body 13 formedwith the inner protective layer 18 is washed with an alkali degreasingagent or the like. Subsequently, the magnet body 13 is dipped in achemical conversion solution and so forth, so as to be subjected tochemical conversion treatment, thus forming a chemical conversion layeron the surface of the second layer 17.

Examples of the chemical conversion solution include aqueous solutionscontaining a metal and an acid ion which constitute the above-mentionedmetal salt. When forming a chemical conversion layer made of a phosphateof the above-mentioned metal as a metal salt layer, for example, achemical conversion solution containing a metal material, phosphoricacid, and an oxidizing agent can be used.

More specifically, employable as the chemical conversion solution whenforming a metal salt layer (chemical conversion layer) made ofmolybdenum phosphate is one containing a molybdate such as sodiummolybdate or molybdic acid as a metal material in combination withphosphoric acid and an oxidizing agent.

Employable as the chemical conversion solution when forming a metal saltlayer made of cerium phosphate is one containing a cerium salt such ascerium nitrate as a metal material in combination with phosphoric acidand an oxidizing agent. Examples of the oxidizing agent contained in thechemical conversion solution include sodium nitrite, sodium nitrate,potassium permanganate, sodium chromate, and hydrogen peroxide.

Though the chemical conversion solution temperature at the time ofchemical conversion treatment is not limited in particular, it will bepreferred from the viewpoint of promoting a reaction between the magnetbody 13 and the chemical conversion solution so as to form the metalsalt layer (chemical conversion layer) in a short time if the chemicalconversion solution is used while being heated to room temperature orhigher. The temperature of the chemical conversion solution ispreferably 30 to 100° C., for example. Though not restricted inparticular, the time (chemical conversion treatment time) for dippingthe magnet body 13 in the chemical conversion solution is preferably 1to 60 min, more preferably 2 to 30 min. When the chemical conversiontreatment time is less than 1 min, the state of formation of thechemical conversion layer tends to become uneven. When the chemicalconversion treatment time exceeds 60 min, the chemical conversion layerbecomes so thick that it becomes less dense, whereby the resultingrare-earth magnet 10 may deteriorate its corrosion resistance and thelike.

Preferably, after the chemical conversion treatment, the surface of theresulting rare-earth magnet 10 is washed with water, so as tosufficiently remove the chemical conversion solution and the likeremaining on the surface, and then the rare-earth magnet 10 is fullydried by heating or the like. When the drying is insufficient, themoisture attached to the surface may cause corrosion in the rare-earthmagnet 10. However, it will be preferred if the heating temperature atthe time of drying is such that characteristics of the rare-earth magnet10 are not deteriorated thereby.

When performed for a support containing a metal element, theabove-mentioned chemical conversion treatment usually advances as themetal element in the support dissolves, thereby forming a stablechemical conversion layer. When directly forming a chemical conversionlayer on the surface of a magnet body such as one based on R-TM-B, arare-earth-rich phase in the above-mentioned magnet body is selectivelydissolved, whereby there has conventionally been a tendency of thechemical conversion layer to be formed insufficiently. However, suchselective dissolution of the rare-earth-rich phase is very hard to occurin the above-mentioned embodiment, since the chemical conversiontreatment is performed after the inner protective layer 18 is formed onthe surface of the magnet body 13 containing a rare-earth element. Thus,a stable metal salt layer (chemical conversion layer) is formed on theoutermost layer of the rare-earth magnet 10 in this embodiment.

(4) Layer Containing an Organic-Inorganic Hybrid Compound

The outer protective layer 19 containing an organic-inorganic hybridcompound is formed so as to cover the inner protective layer 18 (secondlayer 17).

The organic-inorganic hybrid compound contained in the organic-inorganichybrid layer is a compound containing a structural unit made of anorganic polymer and a structural unit made of an inorganic polymer. Inthe following, the “structural unit made of an organic polymer” and“structural unit made of an inorganic polymer” will be referred to as“organic structural unit” and “inorganic structural unit”, respectively,if necessary for convenience of explanation.

An example of the organic structural unit is a polymer structure havinga main chain constructed by bonding carbon atoms to each other. A partof the main chain may include an atom other than carbon, e.g., oxygenatom or nitrogen atom. Such an organic structural unit is not restrictedin particular as long as it is a polymer structure formed from anorganic compound, examples of which include polymer structures oforganic compounds formed by various polymerization reactions such asaddition polymerization, polycondensation, and polyaddition. Preferredamong them are vinyl polymer structures formed from vinyl-containingmonomers and epoxy polymer structures obtained from epoxy-containingmonomers.

An example of the inorganic structural unit is a polymer structurehaving a main chain constructed by an element other than carbon atoms.Preferably, this main chain contains a metal atom as an element otherthan carbon and has a structure in which metal atoms and oxygen atomsare bonded alternately. Preferred as the metal atom in the main chain ofthe inorganic structural unit is Si, Al, Ti, Zr, Ta, Mo, Nb, or B.

Among them, polymer structures having a main chain containing an —Si—O—bond such as polysiloxane structures in particular are preferred inparticular as polymer structures constructing the main chain in theorganic structural unit, since they can be synthesized relatively easilyand can form polymers having various structures. Preferred in particularas the polymer structure having the main chain including the —Si—O— bondis a polymer structure obtained by condensing or cocondensing a compoundrepresented by the following formula (2) and/or its hydrolysate. Sincethe inorganic structural unit made of such a polymer structure isexcellent in stress relaxation, the protective layer containing theorganic-inorganic hybrid compound including this structure is harder togenerate cracks and the like.

[Chemical Formula 2]

R²¹ _(m)Si(OR²²)_(4-m)  (2)

In the above-mentioned formula, R¹¹ is an organic group having a carbonnumber of 1 to 8, R²² is an alkyl group having a carbon number of 1 to 5or an acyl group having a carbon number of 1 to 4, and m is 1 or 2. Whena plurality of R²¹ or R²² exist, they may be identical to or differentfrom each other.

Examples of the organic-inorganic hybrid compounds include compounds inwhich organic and inorganic structural units are combined together bycovalent bonds, compounds in which organic and inorganic structuralunits are combined together by hydrogen bonds, and compounds in which anorganic structural unit having an aromatic ring and an inorganicstructural unit having an aromatic ring are combined together by aninteraction between the aromatic rings. These organic-inorganic hybridcompounds will now be explained individually.

First, an organic-inorganic hybrid compound in which organic andinorganic structural units are combined together by a covalent bond willbe explained.

The covalent bond between the organic and inorganic structural units ismainly a bond between a carbon atom in the organic structural unit and ametal atom in the inorganic structural unit. This covalent bond may beone in which the above-mentioned carbon and metal atoms are directlycombined to each other or one in which the carbon and metal atoms arecombined to each other through another element. In the latter case, onlya covalent bond is formed between the carbon atom and the metal element.Preferred in particular as the covalent bond in the organic-inorganichybrid compound is the former one in which the carbon and metal atomsare directly combined to each other.

Such an organic-inorganic hybrid compound can be formed by the followingmethod, for example. Namely, an example of the method prepares anorganic polymer compound and an inorganic compound which have respectivefunctional groups condensable to each other, and causes a condensationreaction between the organic polymer compound and inorganic compound,while generating a condensation reaction in the inorganic compound, soas to form a polymer, thereby yielding an organic-inorganic hybridcompound having an organic structural unit and an inorganic structuralunit.

Examples of combinations of condensable functional groups in the organicpolymer compound and inorganic compound in such a manufacturing methodinclude combinations of hydroxyl and alkoxy groups and combinations ofhydroxyl groups. Both may have alkoxy groups. In this case, forming ahydroxyl group by hydrolyzing one alkoxy group can generate theabove-mentioned condensation.

When the organic polymer compound partly has a functional grouprepresented by -M¹-OR (where M¹ is a metal element), while the inorganiccompound has a functional group represented by -M²-OR, theirhydrolysis-condensation reaction generates a bond represented by-M¹-O-M²-. Also, functional groups represented by -M²-OR in theinorganic compound cause a condensation reaction, thereby forming aninorganic structural unit. As a result, an organic-inorganic hybridcompound in which the organic and inorganic structural units arecombined together by a covalent bond is obtained. In view of easiness inthe condensation reaction, availability, and so forth, Si is preferredin particular as metal elements represented by M¹ and M².

The outer protective layer 19 containing such an organic-inorganichybrid compound can be formed, for example, by preparing a solutioncontaining the above-mentioned organic polymer compound and inorganiccompound, applying it onto the surface of the inner protective layer 18,and then heating it or leaving it in the air, so as to cause apolymerization reaction (e.g., condensation reaction) of the inorganiccompound. The outer protective layer 19 may also be formed by making anorganic-inorganic hybrid compound beforehand, and applying it onto thesurface of the inner protective layer 18.

The organic-inorganic hybrid compound in which a structural unit made ofan organic polymer and a structural unit made of an inorganic polymerare combined together by a hydrogen bond will now be explained.

Here, the “hydrogen bond” refers to a bond formed by hydrogen interposedbetween two atoms, which is represented by X—H•••Y in general. X and Yrepresent two atoms combined to each other by the hydrogen bond, whereasX—H indicates the covalent bond between the X atom and hydrogen. Namely,the hydrogen bond is formed between the group represented by X—H and theY atom. From such a viewpoint, this organic-inorganic hybrid compoundcan be regarded as one in which organic and inorganic polymers, whichare respective molecules different from each other, are combinedtogether by a hydrogen bond.

For forming a hydrogen bond, the organic and inorganic structural unitshave functional groups which can form a hydrogen bond with each other intheir molecules. Here, an example of functional groups which can form ahydrogen bond includes a combination of a proton-donating functionalgroup (group represented by the above-mentioned X—H) which donateshydrogen in the hydrogen bond and a proton-accepting functional group(group containing the above-mentioned Y) which accepts hydrogen in thehydrogen bond.

Though the organic and inorganic structural units may each have eitherthe proton-donating or proton-accepting functional group, it will bepreferred if the organic and inorganic structural units haveproton-accepting and proton-donating functional groups, respectively.

Examples of the proton-accepting functional group in the organicstructural unit include functional groups having stronglyelectronegative oxygen atoms, nitrogen atoms, fluorine atoms, chlorineatoms, and the like. Specifically, amido, imido, carbonate, and urethanegroups are preferred. Amido group is preferred in particular, since itcan exhibit a high proton acceptability when forming a hydrogen bond.Specific examples of such an organic structural unit (organic polymer)include polyvinylpyrrolidone, polyoxazoline, polyacrylamide derivatives,poly(N-vinylcaprolactone), polyvinylacetamide, and nylon derivatives.

Examples of the proton-donating functional group in the inorganicstructural unit include functional groups having structures representedby —OH and —NH. Specific examples of functional groups having suchstructures include hydroxyl and amino groups. Among them, the hydroxylgroup is preferred in particular, since it can favorably form a hydrogenbond with the above-mentioned proton-accepting functional group.

Preferred as such an inorganic structural unit is a polymer structureobtained by condensing or cocondensing a compound represented by theabove-mentioned formula (2) and/or its hydrolysate, while a hydroxylgroup generated by hydrolyzing an alkoxy group represented by —OR²² inthe above-mentioned condensation or cocondensation reaction exists inthe structure. This yields polysiloxane, whose main chain is constructedby —Si—O— bond, having a hydroxyl group which is a proton-donatingfunctional group.

Whether a hydrogen bond is formed between the organic and inorganicstructural units or not can be verified by a Fourier-transform infraredspectrometer (FT-IR), for example. Specifically, when a hydrogen bond isformed, the functional group contributing to the hydrogen bond usuallyexhibits absorption at a position shifted from an absorption wave numberobtained in the state not contributing to the hydrogen bond in the casewhere a peeled piece of the outer protective layer 19 is analyzed byFT-IR.

Such an organic-inorganic hybrid compound is formed, for example, by amethod preparing an organic polymer compound having a proton-acceptingfunctional group and an inorganic compound having a proton-donatingfunctional group, mixing them, and then causing polymerization of theinorganic compound, so as to yield an organic-inorganic hybrid compoundhaving organic and inorganic structural units. In this case, theinorganic compound may be one having a functional group such as theabove-mentioned alkoxy group which becomes a proton-donating functionalgroup after a reaction such as hydrolysis.

Examples of the organic polymer compound used in this manufacturingmethod include polyvinylpyrrolidone, polyoxazoline, polyacrylamidederivatives, poly(N-vinylcaprolactone), polyvinylacetamide, and nylonderivatives which can form the above-mentioned organic structural unit.An example of the inorganic compound is the compound represented by theabove-mentioned general formula (1).

The outer protective layer 19 containing such an organic-inorganichybrid compound can be formed, for example, by preparing a solutioncontaining the above-mentioned organic polymer compound and inorganiccompound, applying it onto the surface of the inner protective layer 18,and then heating it or leaving it in the air, so as to cause apolymerization reaction (e.g., condensation reaction) of the inorganiccompound. The outer protective layer 19 may also be formed by making anorganic-inorganic hybrid compound beforehand, and applying it onto thesurface of the inner protective layer 18.

An organic-inorganic hybrid compound in which an organic structural unithaving an aromatic ring and an inorganic structural unit having anaromatic ring are combined together by an interaction between thearomatic rings will now be explained.

The aromatic ring is a generic term of rings belonging to aromaticseries, and refers to a thermodynamically stable annular structure inwhich π-electrons are delocalized, such as benzene ring, condensedbenzene rings, non-benzene aromatic rings, and heterocyclic aromaticrings, for example. Among them, the benzene ring is preferred as anaromatic ring in the organic and inorganic structural units.

This organic-inorganic hybrid compound is one in which the organic andinorganic structural units are weakly combined together by aninteraction of π-electrons in the respective aromatic rings (π-πinteraction). From such a viewpoint, this organic-inorganic hybridcompound can be regarded as one in which organic and inorganic polymerswhich are molecules separate from each other are combined together bythe π-π interaction.

The organic structural unit (organic polymer) having such an aromaticring may have the aromatic ring in either its main or side chain, andcan employ both of thermoplastic and thermosetting organic polymers.Examples of the thermoplastic organic polymer include polystyrene,polyester, polyphenylene ether, polysulfone, polyethersulfone,polyphthalamide, polyphenylenesulfide, polyallylate, polyimide,polyamideimide, and polyetherimide. Examples of the thermosettingorganic polymer compound include phenol, epoxy, acrylic, melamine,alkyd, and urea resins having at least one aromatic ring in a repeatingstructural unit.

The inorganic structural unit (inorganic polymer) having an aromaticring may have the aromatic ring in either its main or side chain, apreferred example of which is a polymer structure obtained by condensingor cocondensing a compound represented by the above-mentioned formula(2) and/or its hydrolysate, while at least one group represented by R²¹is a group having an aromatic ring. It will be preferred if thisaromatic ring is introduced as the form of benzyl, β-phenethyl,p-toluoyl, mesityl, p-stynyl, or phenyl group in the compound of theabove-mentioned formula (2).

The outer protective layer 19 containing such an organic-inorganichybrid compound can be formed, for example, by preparing a solutioncontaining the above-mentioned organic polymer compound and inorganiccompound, applying it onto the surface of the inner protective layer 18,and then heating it or leaving it in the air, so as to cause apolymerization reaction (e.g., condensation reaction) of the inorganiccompound. The outer protective layer 19 may also be formed by making anorganic-inorganic hybrid compound beforehand, and applying it onto thesurface of the inner protective layer 18.

Inorganic Additive

The outer protective layer 19, which is any of the above-mentioned oxidelayer, resin layer, metal salt layer, or layer containing anorganic-inorganic hybrid compound, may further contain inorganicadditives in addition to these constituent materials. The outerprotective layer 19 thus containing an inorganic additive attains a moreexcellent heat resistance, and also becomes excellent in terms ofstrength.

Such an inorganic additive is preferably an inorganic additive having aplanar structure (planar inorganic additive), and is preferablyinsoluble to the above-mentioned organic-inorganic hybrid compounds andresins or solvents and the like used when forming the outer protectivelayer.

Examples of constituent materials for such an inorganic additive includetalc, silica, titania, alumina, carbon black (CB), zinc oxide (ZnO),magnesium silicate (MgSiO), and barium sulfate (BaSO₄). The inorganicadditive content in the outer protective layer 19 is preferably 1 to 30mass % in the total mass of the outer protective layer 19.

While the rare-earth magnet 10 in accordance with a preferred embodimentand its manufacturing methods are explained in the foregoing, the innerprotective layer 18 made of the first layer 16 and second layer 17 inthe rare-earth magnet 10 having such a structure is formed by changingthe surface of the magnet body 13, and thus initially hascharacteristics of having a dense structure and being excellent inadhesion to the magnet body 13. This can favorably lower influences ofoutside air such as moisture on the magnet body 13. The outer protectivelayer 19 covering the inner protective layer 18 is a stable layerseparately provided on the surface of the magnet body 13 (second layer17), and thus can exhibit an excellent heat resistance which is hard tobe obtained by layers derived from the magnet body 13.

Though single-layer oxide layers obtained by oxidizing the surface ofthe magnet body, resin layers formed by coating or the like on thesurface of the magnet body, and the like have conventionally been knownas protective layers of rare-earth magnets, the single-layer oxide layeris harder to attain a sufficient corrosion resistance by itself, and theresin layer is harder to attain a sufficient heat resistance (such aheat resistance as to endure a temperature exceeding about 120° C.). Bycontrast, the rare-earth magnet 10 in accordance with the secondembodiment is equipped with the protective layer 15 including theabove-mentioned inner protective layer 18 and outer protective layer 19and thus not only is superior to the rare-earth magnets equipped withthe above-mentioned conventional protective layers, but also has such aheat resistance as to be able to endure a high temperature of about 200°C. required in the use of motors in hybrid cars and the like.

The rare-earth magnet of the second embodiment is not limited to thatmentioned above, but may be modified as appropriate. For example, thoughthe above-mentioned embodiment exemplifies the inner protective layer 18by one having a two-layer structure comprising the first layer 16 andsecond layer 17, it is not restrictive, whereby the inner protectivelayer 18 may have a one-layer structure. An example of the one-layerinner protective layer 18 is an oxide layer formed by oxidizing thesurface of the magnet body 13. An example of such an oxide layer is alayer containing a rare-earth element derived from a magnet body and/ora transition element and an oxygen atom.

EXAMPLES

In the following, the present invention will be explained in furtherdetail with reference to Examples, which do not restrict the presentinvention.

Example A Example 1A

An ingot having a composition of 14.7Nd-77.6Fe-1.6Co-6.1B (numbersindicating atom percent) was made by powder metallurgy, and then wasroughly pulverized. Thereafter, jet mill pulverization with an inert gaswas performed, so as to yield a fine powder having an average particlesize of about 3.5 μm. Thus obtained fine powder was put into a die, andwas molded in a magnetic field. Subsequently, it was sintered in vacuum,and then was heat-treated, so as to yield a sintered body. The resultingsintered body was cut into a size of 20 mm×10 mm×2 mm, and then wasbarrel-polished, so as to yield a magnet body processed into a practicalform.

Next, thus obtained magnet body was dipped in a 2% aqueous HNO₃solution, and then was ultrasonically washed with water.

The magnet body subjected to pickling (acid treatment) as mentionedabove was heat-treated for 10 minutes at 450° C. in an nitrogenatmosphere with a steam partial pressure of 475 hPa, so as to form aprotective layer, thereby yielding a rare-earth magnet.

Using a focused ion beam processing machine, a processed cross sectionwas made at a fractured surface of the rare-earth magnet in which theprotective layer was formed on the surface of the magnet body asmentioned above, and the film structure near the surface was observedwith a scanning electron microscope. Employed as the scanning electronmicroscope was S-4700 manufactured by Hitachi, Ltd. FIG. 5 shows thusobtained electron micrograph, whereas FIG. 6 is a photograph partlyenlarging the electron micrograph of FIG. 5.

It was verified in FIGS. 5 and 6 that the white layer was aplatinum-palladium film for analysis, whereas a second layer having anaverage thickness of 100 nm was formed on the uppermost surface of therare-earth magnet on the lower side of the white layer. It was alsoverified that a first layer having an average thickness of 3 μm wasformed on the lower side of the second layer. Also, as can be seen fromFIG. 5, it was verified that the first layer was formed on the magnetbody, whereas the second layer was formed on the first layer.

Next, this rare-earth magnet was processed into a thin piece by usingthe focused ion beam processing machine, the film structure near thesurface was observed with a transmission electron microscope (JEM-3010manufactured by JEOL Ltd.), and elements contained in the first andsecond layers were analyzed by EDS (Voyager III manufactured by NoranInstruments Inc.). As a result, Nd, Fe, and O were detected as maincomponents from the first layer, whereas no Nd was detected from thesecond layer although Fe and O were detected therefrom.

Also, thus obtained rare-earth magnet was subjected to a pressure cookertest. The test condition was such that the rare-earth magnet was leftfor 100 hr in an environment at 120° C., 0.2 MPa, and 100% RH. As aresult, no changes due to the test were seen in appearance, and nochanges in magnetic flux were observed between before and after thetest.

Thus obtained rare-earth magnet was magnetized, and then was subjectedto a test (ATF immersion test) in which it was dipped in acommercially-available automatic transmission fluid (ATF) for hybridcars having 0.2% of water added thereto, and left therein for 1000 hr at150° C. The tested magnet was magnetized again, and its magnetic fluxwas measured, whereby a deterioration of 1.0% was seen in magnetic fluxas compared with that prior to the test.

Example 2A

A rare-earth magnet having a protective layer was manufactured as inExample 1A except that the heat treatment was performed for 13 min at350° C. in an oxidizing atmosphere with an oxygen concentration of 7%.

When thus obtained rare-earth magnet was observed as in Example 1A, itwas verified that a protective layer comprising a first layer having anaverage thickness of 0.9 μm and a second layer having an averagethickness of 60 nm in this order was formed on the surface of the magnetbody. When this protective layer was analyzed as in Example 1A, Nd, Fe,and O were detected as main components from the first layer, whereas noNd was detected from the second layer although Fe and O were detectedtherefrom.

When thus obtained rare-earth magnet was subjected to a pressure cookertest as in Example 1A, it was verified that the deterioration inmagnetic flux of the rare-earth magnet was 0.2%, which was very small.

Example 3A

A rare-earth magnet having a protective layer was manufactured as inExample 1A except that the heat treatment was performed for 7 min at390° C. in an oxidizing atmosphere with an oxygen concentration of 7%.

When thus obtained rare-earth magnet was observed as in Example 1A, itwas verified that a protective layer comprising a first layer having anaverage thickness of 1 μm and a second layer having an average thicknessof 70 nm in this order was formed on the surface of the magnet body.When this protective layer was analyzed as in Example 1A, Nd, Fe, and Owere detected as main components from the first layer, whereas no Nd wasdetected from the second layer although Fe and O were detectedtherefrom.

When thus obtained rare-earth magnet was subjected to a pressure cookertest as in Example 1A, it was verified that the deterioration inmagnetic flux of the rare-earth magnet was 0.3%, which was very small.

Example 4A

A rare-earth magnet having a protective layer was manufactured as inExample 1A except that the heat treatment was performed for 10 min at410° C. in an oxidizing atmosphere with an oxygen concentration of 0.5%.

When thus obtained rare-earth magnet was observed as in Example 1A, itwas verified that a protective layer comprising a first layer having anaverage thickness of 1.5 μm and a second layer having an averagethickness of 50 nm in this order was formed on the surface of the magnetbody. When this protective layer was analyzed as in Example 1A, Nd, Fe,and O were detected as main components from the first layer, whereas noNd was detected from the second layer although Fe and O were detectedtherefrom.

When thus obtained rare-earth magnet was subjected to a pressure cookertest as in Example 1A, it was verified that the deterioration inmagnetic flux of the rare-earth magnet was 0.3%, which was very small.

Example 5A

A rare-earth magnet having a protective layer was manufactured as inExample 1A except that the heat treatment was performed for 10 min at410° C. in an oxidizing atmosphere with an oxygen concentration of 21%.

When thus obtained rare-earth magnet was observed as in Example 1A, itwas verified that a protective layer comprising a first layer having anaverage thickness of 2.1 μm and a second layer having an averagethickness of 100 nm in this order was formed on the surface of themagnet body. When this protective layer was analyzed as in Example 1A,Nd, Fe, and O were detected as main components from the first layer,whereas no Nd was detected from the second layer although Fe and O weredetected therefrom.

When thus obtained rare-earth magnet was subjected to a pressure cookertest as in Example 1A, it was verified that the deterioration inmagnetic flux of the rare-earth magnet was 0.2%, which was very small.

Example 6A

A rare-earth magnet having a protective layer was manufactured as inExample 1A except that the heat treatment was performed for 10 min at500° C. in an oxidizing atmosphere with an oxygen concentration of 7%.

When thus obtained rare-earth magnet was observed as in Example 1A, itwas verified that a protective layer comprising a first layer having anaverage thickness of 5 μm and a second layer having an average thicknessof 300 nm in this order was formed on the surface of the magnet body.When this protective layer was analyzed as in Example 1A, Nd, Fe, and Owere detected as main components from the first layer, whereas no Nd wasdetected from the second layer although Fe and O were detectedtherefrom.

When thus obtained rare-earth magnet was subjected to a pressure cookertest as in Example 1A, it was verified that the deterioration inmagnetic flux of the rare-earth magnet was 0.3%, which was very small.

Example 7A

A rare-earth magnet having a protective layer was manufactured as inExample 1A except that the heat treatment was performed for 10 min at390° C. in an oxidizing atmosphere with an oxygen concentration of 0.5%and a steam partial pressure of 74 hPa.

When thus obtained rare-earth magnet was observed as in Example 1A, itwas verified that a protective layer comprising a first layer having anaverage thickness of 1.7 μm and a second layer having an averagethickness of 100 nm in this order was formed on the surface of themagnet body. When this protective layer was analyzed as in Example 1A,Nd, Fe, and O were detected as main components from the first layer,whereas no Nd was detected from the second layer although Fe and O weredetected therefrom.

When thus obtained rare-earth magnet was subjected to a pressure cookertest as in Example 1A, it was verified that the deterioration inmagnetic flux of the rare-earth magnet was 0.2%, which was very small.

Example 8A

A rare-earth magnet having a protective layer was manufactured as inExample 1A except that the heat treatment was performed for 10 min at390° C. in an oxidizing atmosphere with an oxygen concentration of 0.5%and a steam partial pressure of 12 hPa.

When thus obtained rare-earth magnet was observed as in Example 1A, itwas verified that a protective layer comprising a first layer having anaverage thickness of 1.4 μm and a second layer having an averagethickness of 80 nm in this order was formed on the surface of the magnetbody. When this protective layer was analyzed as in Example 1A, Nd, Fe,and O were detected as main components from the first layer, whereas noNd was detected from the second layer although Fe and O were detectedtherefrom.

When thus obtained rare-earth magnet was subjected to a pressure cookertest as in Example 1A, it was verified that the deterioration inmagnetic flux of the rare-earth magnet was 0.2%, which was very small.

Example 9A

A rare-earth magnet having a protective layer was manufactured as inExample 1A except that the heat treatment was performed for 10 min at400° C. in an oxidizing atmosphere with a steam partial pressure of 2000hPa.

When thus obtained rare-earth magnet was observed as in Example 1A, itwas verified that a protective layer comprising a first layer having anaverage thickness of 1.8 μm and a second layer having an averagethickness of 120 nm in this order was formed on the surface of themagnet body. When this protective layer was analyzed as in Example 1A,Nd, Fe, and O were detected as main components from the first layer,whereas no Nd was detected from the second layer although Fe and O weredetected therefrom.

When thus obtained rare-earth magnet was subjected to a pressure cookertest as in Example 1A, it was verified that the deterioration inmagnetic flux of the rare-earth magnet was 0.3%, which was very small.

Example 10A

A rare-earth magnet having a protective layer was manufactured as inExample 1A except that the heat treatment was performed for 10 min at330° C. in an oxidizing atmosphere with an oxygen concentration of 7%.

The structure near the surface of thus obtained rare-earth magnet wasanalyzed by depth analysis according to Auger electron spectroscopy. Forthe electron spectroscopy, SAM680 manufactured by ULVAC-PHI, Inc. wasused. As a result, it was verified that a second layer containing Fe andO with no Nd detected was formed by a depth of 16 nm from the surface,whereas a first layer containing Nd, Fe, and O was formed by 0.4 μm onthe lower side of the second layer.

When thus obtained rare-earth magnet was subjected to a pressure cookertest as in Example 1A, it was verified that the deterioration inmagnetic flux of the rare-earth magnet was 0.2%, which was very small.

Example 11A

A rare-earth magnet having a protective layer was manufactured as inExample 1A except that the heat treatment was performed for 10 min at290° C. in an oxidizing atmosphere with an oxygen concentration of 21%.

The structure near the surface of thus obtained rare-earth magnet wasanalyzed by the same method as with Example 10A. As a result, it wasverified that a second layer containing Fe and O with no Nd detected wasformed by a depth of 10 nm from the surface, whereas a first layercontaining Nd, Fe, and O was formed by 0.1 μm on the lower side of thesecond layer.

When thus obtained rare-earth magnet was subjected to a pressure cookertest as in Example 1A, it was verified that the deterioration inmagnetic flux of the rare-earth magnet was 0.3%, which was very small.

Comparative Example 1A

A magnet body was made as in Example 1, and then was pickled with a 2%aqueous HNO₃ solution.

This magnet body was observed with the scanning electron microscope asin Example 1A. FIG. 7 shows thus obtained electron micrograph, whereasFIG. 8 is a photograph partly enlarging the electron micrograph of FIG.7. In FIGS. 7 and 8, the white layer was a platinum-palladium film foranalysis, whereas a magnet body was seen on the lower side of the whitelayer.

Next, without effecting heat treatment in a steam atmosphere, thusobtained magnet body was subjected to the pressure cooker test as inExample 1A. As a result, its appearance changed from silver to black,and a deterioration of 2.1% in magnetic flux was seen.

Thus obtained magnet body was magnetized, and then was subjected to theATF immersion test as in Example 1A. The tested magnet was magnetizedagain, and its magnetic flux was measured, whereby a deterioration of7.5% in magnetic flux was seen in the magnet of Comparative Example 1Aas compared with that prior to the test. Thus, while the magnet ofExample 1A exhibited a deterioration of only 1.0% in magnetic fluxbetween before and after the ATF immersion test, the magnet ofComparative Example 1A showed a deterioration of 7.5% in magnetic flux,whereby it was verified that this magnet exhibited a very largedeterioration in magnetic flux between before and after the ATFimmersion test.

Comparative Example 2A

A rare-earth magnet having a protective layer was manufactured as inExample 1A except that the heat treatment was performed for 10 min at200° C. in an oxidizing atmosphere with an oxygen concentration of 7.0%and a steam partial pressure of 0.5 hPa.

When thus obtained rare-earth magnet was observed as in Example 1A, itwas verified that a protective layer made of only a single layer havingan average thickness of 20 nm was formed on the surface of the magnetbody. When this protective layer was analyzed as in Example 1A, Nd, Fe,and O were detected as main components.

Thus obtained rare-earth magnet was subjected to the pressure cookertest as in Example 1A, whereby it was verified that the deterioration inmagnetic flux of the rare-earth magnet was 0.4%.

Further, thus obtained magnet body was magnetized, and then wassubjected to the ATF immersion test as in Example 1A. The tested magnetwas magnetized again, and its magnetic flux was measured, whereby adeterioration of 4.7% in magnetic flux was seen in the magnet ofComparative Example 1A as compared with that prior to the test. Thus,while the magnet of Example 1A exhibited a deterioration of only 1.0% inmagnetic flux between before and after the ATF immersion test, themagnet of Comparative Example 2A showed a deterioration of 4.7% inmagnetic flux, whereby it was verified that this magnet exhibited a verylarge deterioration in magnetic flux between before and after the ATFimmersion test.

Example B Manufacture of Rare-Earth Magnet Example 1B

An ingot having a composition of 13.2Nd-1.5Dy-77.6Fe-1.6Co-6.1B (numbersindicating atom percent) was made by powder metallurgy, and then wasroughly pulverized. Thereafter, jet mill pulverization with an inert gaswas performed, so as to yield a fine powder having an average particlesize of about 3.5 μm. Thus obtained fine powder was put into a die, andwas molded in a magnetic field. Subsequently, it was sintered in vacuum,and then was heat-treated, so as to yield a sintered body. The resultingsintered body was cut into a size of 35 mm×19 mm×6.5 mm, so as to yielda magnet body processed into a practical form.

Next, thus obtained magnet body was dipped in a 2% aqueous HNO₃solution, and then was ultrasonically washed with water. Subsequently,the magnet body subjected to pickling (acid treatment) was heat-treatedfor 8 min at 450° C. in an oxygen-nitrogen mixed atmosphere with anoxygen partial pressure of 70 hPa (oxygen concentration of 7%).

Thereafter, the magnet body was fixed within a vacuum film-formingchamber, which was evacuated until its degree of vacuum became 1×10⁻³ Paor less. Subsequently, using vacuum deposition, which is a vapor-phasegrowth method, an oxide layer made of aluminum oxide (alumina) wasformed on the magnet body surface such as to yield a thickness of 5 μm.

Specifically, this oxide layer was formed by irradiating aluminum oxideparticles (having a particle size of about 2 to 3 mm) with an electronbeam, so as to melt and evaporate them at the same time. The appliedvoltage and current value at the time of generating the electron beamwere 5 kV and 200 mA, respectively. During when forming the oxide layer,an oxygen gas was circulated at a flow rate of 1.0 sccm within thevacuum film-forming chamber, while the pressure within the chamber wasmaintained at 1×10⁻² Pa. The surface temperature of the magnet body atthis time was adjusted so as to become 200° C., and the film-formingrate of 0.4 nm/sec was maintained. Thus, a rare-earth magnet of Example1 was obtained.

Thus obtained rare-earth magnet was processed into a thin piece by usinga focused ion beam processing machine, and the film structure near itssurface was observed with a transmission electron microscope (JEM-3010manufactured by JEOL Ltd.), whereby it was verified that two layerscomposed of a layer having an average thickness of 1 μm and a layerhaving an average thickness of 50 nm were formed between the magnet bodyand oxide layer on the surface of the magnet body successively from themagnet body side. When elements contained in these two layers wereanalyzed by EDS (Voyager III manufactured by Noran Instruments Inc.),Nd, Fe, and O were detected as main components from the layer on themagnet body side, whereas no Nd was detected from the layer on the oxidelayer side although Fe and O were detected therefrom.

Example 2B

First, as in Example 1B, a magnet body was manufactured and then waspickled. Subsequently, this magnet body was heat-treated for 10 min at390° C. in an oxidizing atmosphere at an oxygen concentration of 0.5%and a steam partial pressure of 74 hPa.

Next, this magnet body was placed in an atmospheric thermal CVD system.This atmospheric thermal CVD system is one which can form a metal oxidelayer on a magnet body by introducing a metal alkoxide to become adeposition source and steam into a reaction furnace with a carrier gassuch as nitrogen gas

Then, Mo(OC₂H₅)₅, Ti(O-i-C₃H₇)₄, and water heated to 60° C. wereemployed as deposition sources and were fed by a carrier gas at 200cm³/min to the magnet body heated to 200° C. This formed a mixed oxidelayer made of molybdenum oxide and titanium oxide having a thickness of0.1 μm on the surface of the magnet body. Thus, a rare-earth magnet ofExample 2B was obtained.

When thus obtained rare-earth magnet was observed with the transmissionelectron microscope as in Example 1B, it was verified that two layerscomposed of a layer having an average thickness of 1.7 μm and a layerhaving an average thickness of 100 nm were formed between the magnetbody and oxide layer on the surface of the magnet body successively fromthe magnet body side. When elements contained in these two layers wereanalyzed by EDS, Nd, Fe, and O were detected as main components from thelayer on the magnet body side, whereas no Nd was detected from the layeron the oxide layer side although Fe and O were detected therefrom. Whenthe mixed oxide layer formed on the surface of the rare-earth magnet wassubjected to fluorescent x-ray analysis, the metal ratio within thelayer was composed of 3 atom % of Mo and 97 atom % of Ti.

Example 3B

First, as in Example 1B, a magnet body was manufactured and then waspickled. Further, this magnet body was heat-treated under the samecondition as that in Example 1B.

Then, Cr(C₅H₇O₂)₃ and water heated to 60° C. were used as depositionsources and fed by a carrier gas at 200 cm³/min to the magnet bodyheated to 200° C. This formed an oxide layer made of chromium oxidehaving a thickness of 0.3 μm on the surface of the magnet body. Thus, arare-earth magnet of Example 3B was obtained.

When the semiconductor characteristic of the layer formed on the surfaceof the magnet body was investigated after the above-mentioned heattreatment in this manufacturing method, it was verified that this layerexhibited the n-type semiconductor characteristic. When thesemiconductor characteristic of the oxide layer was similarlyinvestigated, it was verified that this layer exhibited the p-typesemiconductor characteristic.

When thus obtained rare-earth magnet was observed with the transmissionelectron microscope as in Example 1B, it was verified that two layerscomposed of a layer having an average thickness of 1 μm and a layerhaving an average thickness of 50 nm were formed between the magnet bodyand oxide layer on the surface of the magnet body successively from themagnet body side. When elements contained in these two layers wereanalyzed by EDS, Nd, Fe, and O were detected as main components from thelayer on the magnet body side, whereas no Nd was detected from the layeron the oxide layer side although Fe and O were detected therefrom.

Reference Example 1B

An ingot having a composition of 13.2Nd-1.5Dy-77.6Fe-1.6Co-6.1B (numbersindicating atom percent) was made by powder metallurgy, and then wasroughly pulverized. Thereafter, jet mill pulverization with an inert gaswas performed, so as to yield a fine powder having an average particlesize of about 3.5 μm. Thus obtained fine powder was put into a die, andwas molded in a magnetic field. Subsequently, it was sintered in vacuum,and then was heat-treated, so as to yield a sintered body. The resultingsintered body was cut into a size of 35 mm×19 mm×6.5 mm, so as to yielda magnet body processed into a practical form.

Next, thus obtained magnet body was dipped in a 2% aqueous HNO₃solution, and then was ultrasonically-washed with water. Subsequently,the magnet body subjected to pickling (acid treatment) was heat-treatedfor 8 min at 450° C. in an oxygen-nitrogen mixed atmosphere with anoxygen partial pressure of 70 hPa (oxygen concentration of 7%). Thus, arare-earth magnet of Reference Example 1B was obtained.

When thus obtained rare-earth magnet was observed with the transmissionelectron microscope as in Example 1B, it was verified that two layerscomposed of a layer having an average thickness of 1 μm and a layerhaving an average thickness of 50 nm were formed between the magnet bodyand oxide layer on the surface of the magnet body successively from themagnet body side. When elements contained in these two layers wereanalyzed by EDS, Nd, Fe, and O were detected as main components from thelayer on the magnet body side, whereas no Nd was detected from the layeron the oxide layer side although Fe and O were detected therefrom.

Comparative Example 1B

First, as in Example 1B, a magnet body was manufactured, then dipped ina 2% aqueous HNO₃ solution, and thereafter ultrasonically washed withwater. Subsequently, an acrylic resin paint was applied onto the surfaceof thus pickled (acid-treated) magnet body by a thickness of 10 μm, soas to form a protective layer. Thus, a rare-earth magnet of ComparativeExample 1B was obtained.

Characteristic Evaluation

Salt Spray Test

The rare-earth magnets of Examples 1B to 3B, Reference Example 1B, andComparative Example 1B were subjected to a salt spray test for 96 hr at35° C. with 5% brine in conformity to JIS K5600-7-1. As a result, norust was seen to occur in the rare-earth magnets of Examples 1B to 3Band Comparative Example 1B but in the rare-earth magnet of ReferenceExample 1B.

Heat Resistance Test

An immersion test in which the rare-earth magnets of Examples 1B to 3B,Reference Example 1B, and Comparative Example 1B were dipped in an ATF(automatic transmission fluid) manufactured by Nippon Oil Corporation at200° C. for 1000 hr was performed. As a result, the deterioration inmagnetic flux after the dipping was 0.2% or less in each of therare-earth magnets of Examples 1B to 3B and Comparative Example 1B but5.2% in the rare-earth magnet of Reference Example 1B.

The foregoing results of salt spray test and heat resistance testverified that the rare-earth magnets of Examples 1B to 3B were excellentin both characteristics of the corrosion resistance and heat resistance.

Example C Example 1C

An ingot having a composition of 13.2Nd-1.5Dy-77.6Fe-1.6Co-6.1B (numbersindicating atom percent) was made by powder metallurgy, and then wasroughly pulverized. Thereafter, jet mill pulverization with an inert gaswas performed, so as to yield a fine powder having an average particlesize of about 3.5 μm. Thus obtained fine powder was put into a die, andwas molded in a magnetic field. Subsequently, it was sintered in vacuum,and then was heat-treated, so as to yield a sintered body. The resultingsintered body was cut into a size of 35 mm×19 mm×6.5 mm, so as to yielda magnet body processed into a practical form.

Next, thus obtained magnet body was dipped in a 2% aqueous HNO₃solution, and then was ultrasonically washed with water.

The magnet body subjected to pickling (acid treatment) as mentionedabove was heat-treated for 8 min at 450° C. in an oxygen-nitrogen mixedatmosphere with an oxygen partial pressure of 70 hPa (oxygenconcentration of 7%), so as to form a protective layer.

The rare-earth magnet in which the protective layer was formed on thesurface of the magnet body as mentioned above was processed into a thinpiece by using a focused ion beam processing machine, and the filmstructure near its surface was observed with a transmission electronmicroscope. JEM-3010 manufactured by JEOL Ltd. was employed as thetransmission electron microscope. FIG. 9 shows thus obtained electronmicrograph, whereas FIG. 10 shows a photograph partly enlarging theelectron micrograph of FIG. 9.

It was verified in FIGS. 9 and 10 that the rightmost black layer was aplatinum-palladium film, whereas the white layer adjacent thereto was asecond layer containing no neodymium and having an average thickness of50 nm in the protective layer of the rare-earth magnet. It was alsoverified that the gray layer (the layer gradually deepening its colorfrom the white boundary to the magnet body) adjacent to the second layerwas a first layer containing neodymium and having an average thicknessof 1 μm. As can be seen from FIGS. 9 and 10, it was verified that thefirst layer was formed on the magnet body, whereas the second layer wasformed on the first layer.

Further, the above-mentioned rare-earth magnet was processed into a thinpiece by using the focused ion beam processing machine, the filmstructure near the surface was observed with a transmission electronmicroscope (JEM-3010 manufactured by JEOL Ltd.), and elements containedin the first and second layers were analyzed by EDS (Voyager IIImanufactured by Noran Instruments Inc.). As a result, Nd, Fe, and O weredetected as main components from the first layer, whereas no Nd wasdetected from the second layer although Fe and O were detectedtherefrom.

The rare-earth magnet thus formed with the protective layer was furthercoated with a phenol resin paint by dip-spin coating, which was thenheated for 20 min at 150° C. This process was repeated twice, so as toform a resin layer of about 3 μm, thereby yielding a rare-earth magnetof Example 1C.

Example 2C

As in Example 1C, a sintered body was made and cut into a size of 35mm×19 mm×6.5 mm, so as to yield a magnet body processed into a practicalform. Subsequently, as in Example 1C, pickling was effected, and heattreatment was performed, so as to form a protective layer. It wasverified in thus obtained rare-earth magnet that the first layer wasformed on the magnet body, whereas the second layer was formed on thefirst layer.

The rare-earth magnet thus formed with the protective layer was furthercoated with a phenol resin paint by spray coating, and was heated for 20min at 150° C. Thus, a resin layer of about 5 μm was formed, whereby arare-earth magnet of Example 2C was obtained.

Comparative Example 1C

As in Example 1C, a magnet body was made and pickled with a 2% aqueousHNO₃ solution, so as to yield a rare-earth magnet of Comparative Example1C. A processed cross section was made in the magnet body by using afocused ion beam processing machine, and was observed with a scanningelectron microscope (S-4700 manufactured by Hitachi, Ltd.). FIG. 11shows thus obtained electron micrograph, whereas FIG. 12 is a photographpartly enlarging the electron micrograph of FIG. 11. In FIGS. 11 and 12,the white layer was a platinum-palladium film for analysis, whereas themagnet body was seen on the lower side of the white layer.

Reference Example 1C

As in Example 1C, a magnet body was made and pickled with a 2% aqueousHNO₃ solution. Subsequently, as in Example 1C, heat treatment waseffected, and a protective layer was formed, whereby a rare-earth magnetof Reference Example 1C was obtained. The rare-earth magnet of ReferenceExample 1C was formed with no protective layer. The rare-earth magnet ofReference Example 1C was observed with the transmission electronmicroscope as in Example 1C. As a result, it was also verified in therare-earth magnet of Reference Example 1C that the protective layer wasconstructed by a second layer having an average thickness of 50 nmformed on the outermost surface of the rare-earth magnet and a firstlayer having an average thickness of 1 μm formed on the lower side ofthe second layer.

Salt Spray Test

The rare-earth magnets of Examples 1C and 2C, Comparative Example 1C,and Reference Example 1C were subjected to a salt spray test for 96 hrat 35° C. with 5% brine in conformity to JIS K5600-7-1.

When the rare-earth magnet magnetic flux was measured in Examples 1C and2C, Comparative Example 1C, and Reference Example 1C after the saltspray test, the magnet flux decreased from that prior to the test by0.4% in Example 1C, 2.7% in Comparative Example 1C, and 2.0% inReference Example 1C. No decrease in magnetic flux was seen in Example2C.

The state of occurrence of rust by the salt spray test was comparedamong the rare-earth magnets of Examples 1C and 2C, Comparative Example1C, and Reference Example 1C. FIGS. 13, 15, and 17 show photographs ofthe rare-earth magnets in accordance with Example 2C, ComparativeExample 1C, and Reference Example 1C, respectively, prior to the saltspray test. At 24 hr after starting the salt spray test, the occurrenceof rust was partial and minor in Example 1C, whereas no rust was seen tooccur in Example 2C. By contrast, the whole magnet was covered with rustin Comparative Example 1C and Reference Example 1C, in which theoccurrence of rust was remarkable in Comparative Example 1C inparticular. FIGS. 14, 16, and 18 show photographs of the rare-earthmagnets in accordance with Example 2C, Comparative Example 1C, andReference Example 1C, respectively, at 24 hr after starting the saltspray test.

The state of occurrence of rust was also compared among the rare-earthmagnets at 96 hr after starting the salt spray test. As a result, rustoccurred in Comparative Example 1C and Reference Example 1C too thick topeel off from the magnet surface, and could not be completely wiped offwhen tried, thereby remaining on the surface. In Example 1C, bycontrast, though rust occurring and flowing from a part with anincomplete resin layer such as a corner of the magnet covered about ahalf of the magnet surface, the rust layer was removed when the rust waswiped off, whereby it was verified that the occurrence of rust wasminor. When the cross section was observed, rust occurred by a thicknessof about 50 μm from the magnet surface in Reference Example 1C. On theother hand, no rust was observed in the cross section in Example 1C.Also, no rust was seen to occur in Example 2C.

Pressure Cooker Test

The rare-earth magnets of Examples 1C and 2C were subjected to apressure cooker test. The test condition was such that they were leftfor 100 hr in an environment at 120° C., 0.2 MPa, and 100% RH. As aresult, in both of Examples 1C and 2C, no exterior changes such as thepeeling of resin layers, swelling, and occurrence of rust by the testwere seen, and no changes in magnetic flux was found between before andafter the test.

Example D Manufacture of Rare-Earth Magnet Reference Example 1D

An ingot having a composition of 13.2Nd-1.5Dy-77.6Fe-1.6Co-6.1B (numbersindicating atom percent) was made by powder metallurgy, and then wasroughly pulverized. Thereafter, jet mill pulverization with an inert gaswas performed, so as to yield a fine powder having an average particlesize of about 3.5 μm. Thus obtained fine powder was put into a die, andwas molded in a magnetic field. Subsequently, it was sintered in vacuum,and then was heat-treated, so as to yield a sintered body. The resultingsintered body was cut into a size of 35 mm×19 mm×6.5 mm, so as to yielda magnet body processed into a practical form.

Next, thus obtained magnet body was dipped for 2 minute in a 2% aqueousHNO₃ solution, and then was ultrasonically washed with water.Subsequently, the magnet body subjected to pickling (acid treatment) washeat-treated for 8 min at 450° C. in an oxygen-nitrogen mixed atmospherewith an oxygen partial pressure of 70 hPa (oxygen concentration of 7%).

Thereafter, the heat-treated magnet body was dipped in a chemicalconversion solution containing 0.1-M sodium molybdate, 1.0-M phosphoricacid, and 0.05-M sodium nitrite at 70° C. for 10 min, so as to performchemical conversion treatment of the magnet body, thereby forming achemical conversion layer on the surface.

Thus obtained rare-earth magnet was processed into a thin piece by usinga focused ion beam processing machine, and the film structure near itssurface was observed with a transmission electron microscope (JEM-3010manufactured by JEOL Ltd.), whereby it was verified that two layerscomposed of a layer having an average thickness of 2.5 μm and a layerhaving an average thickness of 80 nm were formed between the magnet bodyand chemical conversion layer on the surface of the magnet bodysuccessively from the magnet body side. When elements contained in thesetwo layers were analyzed by EDS (Voyager III manufactured by NoranInstruments Inc.), Nd, Fe, and O were detected as main components fromthe layer on the magnet body side, whereas no Nd was detected from thelayer on the chemical conversion layer side although Fe and O weredetected therefrom.

Example 2D

First, as in Example 1D, a magnet body was manufactured and then waspickled. Subsequently, heat treatment was performed for 8 min at 450° C.in an oxygen-nitrogen mixed atmosphere with an oxygen partial pressureof 70 hPa (oxygen concentration of 7%). Here, the structure near thesurface of thus obtained rare-earth magnet was analyzed by depthanalysis according to Auger electron spectroscopy. For the electronspectroscopy, SAM680 manufactured by ULVAC-PHI, Inc. was used. As aresult, it was verified that a layer containing Fe and O with no Nddetected was formed by a depth of 80 nm from the surface, whereas alayer containing Nd, Fe, and O was formed by 2.5 μm on the lower side ofthe former layer.

Thereafter, the heat-treated magnet body was dipped in a chemicalconversion solution containing 0.1-M cerium nitrate, 1.0-M phosphoricacid, and 0.05-M sodium nitrite at 80° C. for 10 min, so as to performchemical conversion treatment of the magnet body, thereby forming achemical conversion layer on the surface.

Reference Example 1D

As in Example 1D, a magnet body was formed, pickled, and thenheat-treated. Thus obtained rare-earth magnet was employed as arare-earth magnet of Reference Example 1D. When this rare-earth magnetwas observed with the transmission electron microscope as in Example 1,it was verified that two layers composed of a layer having an averagethickness of 2.5 μm and a layer having an average thickness of 80 nmwere formed between the magnet body and oxide layer on the surface ofthe magnet body successively from the magnet body side. When elementscontained in these two layers were analyzed by EDS, Nd, Fe, and O weredetected as main components from the layer on the magnet body side,whereas no Nd was detected from the layer on the oxide layer sidealthough Fe and O were detected therefrom.

[Characteristic Evaluation]

Salt Spray Test

The rare-earth magnets of Examples 1D and 2D and Reference Example 1Dwere subjected to a salt spray test in which 5% brine was sprayed for 96hr at 35° C. in conformity to JIS K5600-7-1. As a result, no rust wasseen to occur in the rare-earth magnets of Examples 1D and 2D but in therare-earth magnet of Reference Example 1D.

Heat Resistance Test

An immersion test in which the rare-earth magnets of Examples 1D and 2Dand Reference Example 1D were dipped in an ATF (automatic transmissionfluid) manufactured by Nippon Oil Corporation at 200° C. for 1000 hr wasperformed. As a result, the deterioration in magnetic flux after thedipping was 0.2% or less in each of the rare-earth magnets of Examples1D and 2D but 5.3% in the rare-earth magnet of Reference Example 1D.

The foregoing results of salt spray test and heat resistance testverified that the rare-earth magnets of Examples 1D and 2D were superiorto the rare-earth magnet of Reference Example 1D in both characteristicsof the corrosion resistance and heat resistance.

Example E Manufacture of Rare-Earth Magnet Example 1E

An ingot having a composition of 13.2Nd-1.5Dy-77.6Fe-1.6Co-6.1B (numbersindicating atom percent) was made by powder metallurgy, and then wasroughly pulverized. Thereafter, jet mill pulverization with an inert gaswas performed, so as to yield a fine powder having an average particlesize of about 3.5 μm. Thus obtained fine powder was put into a die, andwas molded in a magnetic field. Subsequently, it was sintered in vacuum,and then was heat-treated, so as to yield a sintered body. The resultingsintered body was cut into a size of 35 mm×19 mm×6.5 mm, so as to yielda magnet body processed into a practical form.

Next, thus obtained magnet body was dipped in a 2% aqueous HNO₃solution, and then was ultrasonically washed with water. Subsequently,the magnet body subjected to pickling (acid treatment) was heat-treatedfor 8 min at 450° C. in an oxygen-nitrogen mixed atmosphere with anoxygen partial pressure of 70 hPa (oxygen concentration of 7%), so as toform an inner protective layer on the surface of the magnet body.

Thereafter, a composition containing 40 parts by mass of xylene as asolvent and 60 parts by mass of a thermosetting alkyl phenol wasprepared, applied onto the surface of the heat-treated magnet body,dried at normal temperature, and then cured by heating in the air at150° C. for 30 min, so as to form an outer protective layer on thesurface of the inner protective layer, thereby yielding a rare-earthmagnet.

Thus obtained rare-earth magnet was processed into a thin piece by usinga focused ion beam processing machine, and the film structure near itssurface was observed with a transmission electron microscope (JEM-3010manufactured by JEOL Ltd.), whereby it was verified that two layerscomposed of a layer having an average thickness of 1 μm and a layerhaving an average thickness of 50 nm were formed as the inner protectivelayer on the surface of the magnet body successively from the magnetbody side. When elements contained in these two layers were analyzed byEDS (Voyager III manufactured by Noran Instruments Inc.), Nd, Fe, and Owere detected as main components from the layer adjacent to the magnetbody, whereas no Nd was detected from the layer remote from the magnetbody although Fe and O were detected therefrom.

Example 2E

A rare-earth magnet was obtained as in Example 1E except that an alkylpolyhydric phenol (urushiol) was used in place of the thermosettingalkyl phenol.

When the film structure of thus obtained rare-earth magnet was observedas in Example 1E, it was verified that two layers composed of a layerhaving an average thickness of 1 μm and a layer having an averagethickness of 50 nm were formed as the inner protective layer on thesurface of the magnet body successively from the magnet body side. Whenelements contained in these two layers were analyzed by EDS, Nd, Fe, andO were detected as main components from the layer adjacent to the magnetbody, whereas no Nd was detected from the layer remote from the magnetbody although Fe and O were detected therefrom.

Example 3E

A rare-earth magnet was obtained as in Example 1E except that 30 mass %of an epoxy resin (Araldite) was further added as a material for formingthe outer protective layer.

When the film structure of thus obtained rare-earth magnet was observedas in Example 1E, it was verified that two layers composed of a layerhaving an average thickness of 1 μm and a layer having an averagethickness of 50 nm were formed as the inner protective layer on thesurface of the magnet body successively from the magnet body side. Whenelements contained in these two layers were analyzed by EDS, Nd, Fe, andO were detected as main components from the layer adjacent to the magnetbody, whereas no Nd was detected from the layer remote from the magnetbody although Fe and O were detected therefrom.

Example 4E

First, as in Example 1E, a magnet body was manufactured, and then aninner protective layer was formed on the surface of the magnet body.

Separately from the above, 28 g of methyl methacrylate, 6 g of2-ethylhexyl methacrylate, and 6 g ofγ-methacryloxypropyltrimethoxysilane were added to 40 g of 2-propanoland mixed therewith, and then 1.6 g of 2,2′-azoisobutyronitrile wereadded to the resulting solution and caused to react therewith, so as toprepare a solution of an acrylic resin having a silyl group. When themolecular weight of this acrylic resin was measured by gel permeationchromatography, its weight-average molecular weight was about 10000(calculated by an analytical curve using standard polystyrene).

Next, 80 g of methyltrimethoxysilane, 15 g of 2-propanol, and 17.5 g of0.1% aqueous ammonia were further added to 40 g of the acrylic resinsolution, and they were caused to react for 5 hr at 50° C., whereby acoating liquid containing an organic-inorganic hybrid compound in whichthe acrylic resin and a polymer of methyltrimethoxysilane were combinedtogether was obtained.

Thereafter, this coating liquid was applied by dip coating onto thesurface of the inner protective layer in the above-mentioned magnetbody, and then was heated at 150° C. for 20 min, so as to form an outerprotective layer made of the organic/inorganic hybrid compound, therebyyielding a rare-earth magnet.

When the film structure of thus obtained rare-earth magnet was observedas in Example 1E, it was verified that two layers composed of a layerhaving an average thickness of 1 μm and a layer having an averagethickness of 50 nm were formed as the inner protective layer on thesurface of the magnet body successively from the magnet body side. Whenelements contained in these two layers were analyzed by EDS, Nd, Fe, andO were detected as main components from the layer adjacent to the magnetbody, whereas no Nd was detected from the layer remote from the magnetbody although Fe and O were detected therefrom.

Example 5E

First, as in Example 1E, a magnet body was manufactured, and then aninner protective layer was formed on the surface of the magnet body.

Separately from the above, 20 g of polyvinylidone having aweight-average molecular weight of 40000 were dissolved in 2-propanol,80 g of trimethyltrimethoxysilane and 17.5 g of 0.1% aqueous ammoniawere added to the resulting solution, and then heat treatment wasperformed at 50° C. for 5 hr, so as to cause a polycondensation reactionof methyltrimethoxysilane, thereby preparing a coating liquid. Theweight-average molecular weight was a value calculated with reference toan analysis curve using standard polystyrene after measurement by gelpermeation chromatography.

Thereafter, this coating liquid was applied by dip coating onto thesurface of the inner protective layer in the above-mentioned magnetbody, and then was heated at 150° C. for 20 min, so as to form an outerprotective layer, thereby yielding a rare-earth magnet.

When the film structure of thus obtained rare-earth magnet was observedas in Example 1E, it was verified that two layers composed of a layerhaving an average thickness of 1 μm and a layer having an averagethickness of 50 nm were formed as the inner protective layer on thesurface of the magnet body successively from the magnet body side. Whenelements contained in these two layers were analyzed by EDS, Nd, Fe, andO were detected as main components from the layer adjacent to the magnetbody, whereas no Nd was detected from the layer remote from the magnetbody although Fe and O were detected therefrom.

Example 6E

First, as in Example 1E, a magnet body was manufactured, and then aninner protective layer was formed on the surface of the magnet body.

Separately from the above, 20 g of polystyrene having a weight-averagemolecular weight of 2000 were dissolved in 80 g of tetrahydrofuran, 105g of phenyltrimethoxysilane and 17.5 of 0.1% aqueous ammonia were addedto the resulting solution, and then heat treatment was performed at 50°C. for 5 hr, so as to cause a polycondensation reaction ofphenyltrimethoxysilane, thereby preparing a coating liquid. Theweight-average molecular weight was a value calculated with reference toan analysis curve using standard polystyrene after measurement by gelpermeation chromatography.

Thereafter, this coating liquid was applied by dip coating onto thesurface of the inner protective layer in the above-mentioned magnetbody, and then was heated at 150° C. for 20 min, so as to form an outerprotective layer, thereby yielding a rare-earth magnet.

When the film structure near the surface of thus obtained rare-earthmagnet was observed as in Example 1E, it was verified that two layerscomposed of a layer having an average thickness of 1 μm and a layerhaving an average thickness of 50 nm were formed as the inner protectivelayer on the surface of the magnet body successively from the magnetbody side. When elements contained in these two layers were analyzed byEDS, Nd, Fe, and O were detected as main components from the layeradjacent to the magnet body, whereas no Nd was detected from the layerremote from the magnet body although Fe and O were detected therefrom.

Example 7E

A rare-earth magnet was obtained as in Example 1E except that onefurther containing talc (H₂Mg₃O₁₂Si₄) which was an inorganic additivewas used as a material forming the outer protective layer. Thecompounding amount of talc was such that the content of talc in theouter protective layer was 20 vol %.

Reference Example 1E

As in Example 1E, a magnet body was formed, and then an inner protectivelayer was formed on the surface of the magnet body, whereby theresulting product was used as a rare-earth magnet of Comparative Example1E. When thus obtained rare-earth magnet was observed with thetransmission electron microscope as in Example 1E, it was verified thattwo layers composed of a layer having an average thickness of 1 μm and alayer having an average thickness of 50 nm were formed on the surface ofthe magnet body successively from the magnet body side. When elementscontained in these two layers were analyzed by EDS, Nd, Fe, and O weredetected as main components from the layer adjacent to the magnet body,whereas no Nd was detected from the layer remote from the magnet bodyalthough Fe and O were detected therefrom.

Comparative Example 1E

First, a magnet body was manufactured as in Example 1E. Thereafter,without forming an inner protective layer, a bisphenol-type epoxy resinpaint was applied onto the surface of the magnet body, so as to form aprotective layer having a thickness of 10 μm, thereby yielding arare-earth magnet.

Comparative Example 2E

First, as in Example 1E, a magnet body was manufactured, and then aninner protective layer was formed on the surface of the magnet body.Subsequently, a silicone resin paint (SR2410 manufactured by ToraySilicone Co., Ltd.) was applied onto the surface of the inner protectivelayer, so as to form a protective layer having a thickness of 10 μm,thereby yielding a rare-earth magnet.

Characteristic Evaluation

Salt Spray Test

The rare-earth magnets of Examples 1E to 7E, Reference Example 1E, andComparative Examples 1E and 2E were subjected to a salt spray test inwhich 5% brine was sprayed for 96 hr at 35° C. in conformity to JISK5600-7-1. As a result, no rust was seen to occur in the rare-earthmagnets of Examples 1E to 7E and Comparative Examples 1E and 2E but inthe rare-earth magnet of Reference Example 1E.

Heat Resistance Test

An immersion test in which the rare-earth magnets of Examples 1E to 7E,Reference Example 1E, and Comparative Examples 1E and 2E were dipped inan ATF (automatic transmission fluid) manufactured by Nippon OilCorporation at 120° C. for 500 hr was conducted. As a result, thedeterioration in magnetic flux after the dipping was 0.05% or less ineach of the rare-earth magnets of Examples 1E to 7E and ReferenceExample 1E, whereas peeling of the outer protective layer occurred inComparative Examples 1E and 2E, whereby the magnetic flux deterioratedby 3.2% and 2.4%, respectively, after the immersion.

The foregoing results of salt spray test and heat resistance testverified that the rare-earth magnets of Examples 1E to 7E were excellentin both characteristics of the corrosion resistance and heat resistance.By contrast, it was verified that the rare-earth magnet of ReferenceExample 1E exhibited somewhat low corrosion resistance while beingexcellent in heat resistance, whereas Comparative Examples 1E and 2Eexhibited very low heat resistance while being excellent in corrosionresistance.

1-51. (canceled)
 52. A rare-earth magnet comprising a magnet bodycontaining a rare-earth element, and a protective layer formed on asurface of the magnet body; a protective layer having a first layercovering the magnet body and containing a rare-earth element, and asecond layer covering the first layer and containing substantially norare-earth element.
 53. A rare-earth magnet according to claim 52,wherein the protective layer is formed by heat-treating the magnet bodyin an oxidizing atmosphere containing an oxidizing gas while adjustingat least one condition of a partial pressure of the oxidizing gas, atreatment temperature, and a treatment time such as to have the firstlayer covering the magnet body and containing a rare-earth element, andthe second layer covering the first layer and containing substantial norare-earth element.
 54. A rare-earth magnet comprising a magnet bodycontaining a rare-earth element, and a protective layer formed on asurface of the magnet body; the protective layer having a first layercovering the magnet body and containing a rare-earth element, and asecond layer covering the first layer and containing a rare-earthelement by an amount smaller that that in the first layer.
 55. Arare-earth magnet according to claim 54, wherein the protective layer isformed by heat-treating the magnet body in an oxidizing atmospherecontaining an oxidizing gas while adjusting at least one condition of apartial pressure of the oxidizing gas, a treatment temperature, and atreatment time such as to have the first layer covering the magnet bodyand containing a rare-earth element, and the second layer covering thefirst layer and containing a rare-earth element by an amount smallerthan that in the first layer.
 56. A rare-earth magnet according to claim52, wherein the protective layer contains oxygen and an element derivedfrom the magnet body.
 57. A rare-earth magnet according to claim 52,wherein the magnet body contains a rare-earth element and a transitionelement other than the rare-earth element; wherein the first layercontains the rare-earth element, the transition element, and oxygen; andwherein the second layer contains the transition element and oxygen. 58.A rare-earth magnet according to claim 57, wherein the rare-earthelement in the first layer, the transition element in the first layer,and the transition element in the second layer are elements derived fromthe magnet body.
 59. A rare-earth magnet according to claim 57, whereinthe rare-earth element in the first layer, the transition element in thefirst layer, and the transition element in the second layer are elementsconstructing a main phase of the magnet body.
 60. A rare-earth magnetaccording to claim 52, wherein the rare-earth element is neodymium. 61.A rare-earth magnet according to claim 52, wherein the first and secondlayers have a total thickness of 0.1 to 20 μm.
 62. A rare-earth magnetcomprising a magnet body containing a rare-earth element, and aprotective layer formed on a surface of the magnet body; the protectivelayer having an inner protective layer containing a rare-earth elementand/or a transition element and oxygen, and an outer protective layermade of a constituent material different from that of the innerprotective layer.
 63. A rare-earth magnet according to claim 62, whereinthe inner protective layer has a first layer covering the magnet bodyand containing a rare-earth element, and a second layer covering thefirst layer and containing substantially no rare-earth element.
 64. Arare-earth magnet according to claim 62, wherein the inner protectivelayer has a first layer covering the magnet body and containing arare-earth element, and a second layer covering the first layer andcontaining a rare-earth element by an amount smaller than that in thefirst layer.
 65. A rare-earth magnet according to claim 63, wherein themagnet body contains a rare-earth element and a transition element otherthan the rare-earth element; wherein the first layer contains therare-earth element, the transition element, and oxygen; and wherein thesecond layer contains the transition element and oxygen.
 66. Arare-earth magnet according to claim 65, wherein the rare-earth elementin the first layer, the transition element in the first layer, and thetransition element in the second layer are elements derived from themagnet body.
 67. A rare-earth magnet according to claim 65, wherein therare-earth element in the first layer, the transition element in thefirst layer, and the transition element in the second layer are elementsconstructing a main phase of the magnet body.
 68. A rare-earth magnetaccording to claim 62, wherein the outer protective layer is an oxidelayer having a composition different from that of the inner protectivelayer.
 69. A rare-earth magnet according to claim 68, wherein the oxidelayer contains a metal element different from that contained in theinner protective layer.
 70. A rare-earth magnet according to claim 68,wherein the oxide layer is an amorphous layer.
 71. A rare-earth magnetaccording to claim 68, wherein the oxide layer has a layer made of ap-type oxide semiconductor, and a layer made of an n-type oxidesemiconductor formed on the outer side thereof.
 72. A rare-earth magnetaccording to claim 68, wherein the outer protective layer is an oxidelayer containing an oxide of at least one species of element selectedfrom the group consisting of Al, Ta, Zr, Hf, Nb, P, Si, Ti, Mg, Cr, Ni,Ba, Mo, V, W, Zn, Sr, Bi, B, Ca, Ga, Ge, La, Pb, In, and Mn.
 73. Arare-earth magnet according to claim 68, where the oxide layer containsan oxide of Mo or W.
 74. A rare-earth magnet according to claim 62,wherein the outer protecting layer is a resin layer containing a resin.75. A rare-earth magnet according to claim 74, wherein the resin is athermosetting resin.
 76. A rare-earth magnet according to claim 74,wherein the resin is at least one species of resin selected from thegroup consisting of phenol, epoxy, and melamine resins.
 77. A rare-earthmagnet according to claim 62, wherein the outer protective layer is ametal salt layer.
 78. A rare-earth magnet according to claim 77, whereinthe metal salt layer contains at least one species of element selectedfrom the group consisting of Cr, Ce, Mo, W, Mn, Mg, Zn, Si, Zr, V, Ti,and Fe and at least one species of element selected from the groupconsisting of P, O, C, and S.
 79. A rare-earth magnet according to claim77, wherein the metal salt layer contains at least one species ofelement selected from the group consisting of Mo, Ce, Mg, Zr, Mn, and Wand at least one species of element selected from the group consistingof P, O, C, and S.
 80. A rare-earth magnet according to claim 62,wherein the outer protective layer contains an organic-inorganic hybridcompound having a structural unit made of an organic polymer and astructural unit made of an inorganic polymer, the structural unitshaving a chemical bond therebetween.
 81. A rare-earth magnet accordingto claim 80, wherein the organic-inorganic hybrid compound is a compoundhaving a covalent bond combining the structural unit made of the organicpolymer and the structural unit made of the inorganic polymer together.82. A rare-earth magnet according to claim 80, wherein theorganic-inorganic hybrid compound is a compound having a hydrogen bondcombining the structural unit made of the organic polymer and thestructural unit made of the inorganic polymer together.
 83. A rare-earthmagnet according to claim 80, wherein the organic-inorganic hybridcompound is a compound having the structural unit made of the organicpolymer including an aromatic ring and the structural unit made of theinorganic polymer including an aromatic ring, the structural units beingcombined together by an interaction between the aromatic rings.
 84. Arare-earth magnet according to claim 62, wherein the outer protectivelayer further contains an inorganic additive.
 85. A method ofmanufacturing a rare-earth magnet by forming a protective layer on asurface of a magnet body containing a rare-earth element; the methodcomprising a protective layer forming step of heat-treating the magnetbody so as to form a protective layer having a first layer covering themagnet body and containing a rare-earth element and a second layercovering the first layer and containing substantially no rare-earthelement.
 86. A method of manufacturing a rare-earth magnet by forming aprotective layer on a surface of a magnet body containing a rare-earthelement; the method comprising a protective layer forming step ofheat-treating the magnet body so as to form a protective layer having afirst layer covering the magnet body and containing a rare-earth elementand a second layer covering the first layer and containing a rare-earthelement by an amount smaller than that in the first layer.
 87. A methodof manufacturing a rare-earth magnet according to claim 85, wherein themagnet body is heat-treated in the protective layer forming step in anoxidizing atmosphere containing an oxidizing gas while adjusting atleast one condition of a partial pressure of the oxidizing gas, atreatment temperature, and a treatment time such that the protectivelayer has the first layer and the second layer.
 88. A method ofmanufacturing a rare-earth magnet according to claim 85, furthercomprising a pickling step of pickling the magnet body prior to the heattreatment.
 89. A method of manufacturing a rare-earth magnet accordingto claim 85, wherein the oxidizing atmosphere is a steam atmospherehaving a steam partial pressure of 10 to 2000 hPa.
 90. A method ofmanufacturing a rare-earth magnet according to claim 85, wherein thetreatment time is 1 min to 24 hr.
 91. A method of manufacturing arare-earth magnet by forming a protective layer on a surface of a magnetbody containing a rare-earth element; the method comprising: an innerlayer forming step of heat-treating the magnet body so as to form aninner protective layer covering the magnet body and containing arare-earth element and/or a transition element and oxygen; and an outerprotective layer forming step of forming an outer protective layer madeof a constituent material different from that of the inner protectivelayer on a surface of the inner protective layer.
 92. A method ofmanufacturing a rare-earth magnet according to claim 91, wherein, in theouter protective layer forming step, the magnet body is heat-treated soas to form the inner protective layer having a first layer covering themagnet body and containing a rare-earth element and a second layercovering the first layer and containing substantially no rare-earthelement.
 93. A method of manufacturing a rare-earth magnet according toclaim 91, wherein, in the outer protective layer forming step, themagnet body is heat-treated so as to form the inner protective layerhaving a first layer covering the magnet body and containing arare-earth element by an amount smaller than that in the first layer.94. A method of manufacturing a rare-earth magnet according to claim 92,wherein, in the outer protective layer forming step, the magnet body isheat-treated in an oxidizing atmosphere containing an oxidizing gaswhile adjusting at least one condition of a partial pressure of theoxidizing gas, a treatment temperature, and a treatment time such thatthe protective layer has the first layer and the second layer.
 95. Amethod of manufacturing a rare-earth magnet according to claim 91,wherein, in the outer protective layer forming step, the outerprotective layer made of an oxide layer having a composition differentfrom the inner protective layer is formed on the surface of the innerprotective layer.
 96. A method of manufacturing a rare-earth magnetaccording to claim 91, wherein, in the outer protective layer formingstep, a resin layer forming coating liquid containing a resin is appliedonto the surface of the inner protective layer and dried so as to formthe outer protective layer made of a resin layer.
 97. A method ofmanufacturing a rare-earth magnet according to claim 96, wherein theresin is at least one species of resin selected from the groupconsisting of phenol, epoxy, and melamine resins.
 98. A method ofmanufacturing a rare-earth magnet according to claim 91, wherein, in theouter protective layer forming step, the magnet body after the innerprotective layer forming step is subjected to chemical conversiontreatment so as to form the outer protective layer made of a metal saltlayer containing a metal salt on the surface of the inner protectivelayer.
 99. A method of manufacturing a rare-earth magnet according toclaim 91, wherein, in the outer protective layer forming step, the outerprotective layer made of a layer containing an organic-inorganic hybridcompound having a structural unit made of an organic polymer and astructural unit made of an inorganic polymer is formed on the surface ofthe inner protective layer.
 100. A method of manufacturing a rare-earthmagnet by heat-treating a magnet body containing a rare-earth element soas to form a protective layer on a surface of the magnet body; themethod comprising: a pickling step of pickling the magnet body; and aheat-treating step of heat-treating the picked magnet body in anoxidizing atmosphere containing an oxidizing gas.
 101. A method ofmanufacturing a rare-earth magnet according to claim 100, wherein theheat-treating step is performed subsequent to the pickling step.
 102. Amethod of manufacturing a rare-earth magnet according to claim 100,wherein the magnet body containing an unprocessed part is pickled in thepickling step.
 103. A rare-earth magnet according to claim 53, whereinthe protective layer contains oxygen and an element derived from themagnet body.
 104. A rare-earth magnet according to claim 54, wherein theprotective layer contains oxygen and an element derived from the magnetbody.
 105. A rare-earth magnet according to claim 53, wherein the magnetbody contains a rare-earth element and a transition element other thanthe rare-earth element; wherein the first layer contains the rare-earthelement, the transition element, and oxygen; and wherein the secondlayer contains the transition element and oxygen.
 106. A rare-earthmagnet according to claim 54, wherein the magnet body contains arare-earth element and a transition element other than the rare-earthelement; wherein the first layer contains the rare-earth element, thetransition element, and oxygen; and wherein the second layer containsthe transition element and oxygen.
 107. A rare-earth magnet according toclaim 53, wherein the rare-earth element is neodymium.
 108. A rare-earthmagnet according to claim 54, wherein the rare-earth element isneodymium.
 109. A rare-earth magnet according to claim 53, wherein thefirst and second layers have a total thickness of 0.1 to 20 μm.
 110. Arare-earth magnet according to claim 54, wherein the first and secondlayers have a total thickness of 0.1 to 20 μm.
 111. A rare-earth magnetaccording to claim 64, wherein the magnet body contains a rare-earthelement and a transition element other than the rare-earth element;wherein the first layer contains the rare-earth element, the transitionelement, and oxygen; and wherein the second layer contains thetransition element and oxygen.
 112. A rare-earth magnet according toclaim 63, wherein the outer protective layer is an oxide layer having acomposition different from that of the inner protective layer.
 113. Arare-earth magnet according to claim 64, wherein the outer protectivelayer is an oxide layer having a composition different from that of theinner protective layer.
 114. A rare-earth magnet according to claim 63,wherein the outer protecting layer is a resin layer containing a resin.115. A rare-earth magnet according to claim 64, wherein the outerprotecting layer is a resin layer containing a resin.
 116. A rare-earthmagnet according to claim 63, wherein the outer protective layer is ametal salt layer.
 117. A rare-earth magnet according to claim 64,wherein the outer protective layer is a metal salt layer.
 118. Arare-earth magnet according to claim 63, wherein the outer protectivelayer contains an organic-inorganic hybrid compound having a structuralunit made of an organic polymer and a structural unit made of aninorganic polymer, the structural units having a chemical bondtherebetween.
 119. A rare-earth magnet according to claim 64, whereinthe outer protective layer contains an organic-inorganic hybrid compoundhaving a structural unit made of an organic polymer and a structuralunit made of an inorganic polymer, the structural units having achemical bond therebetween.
 120. A rare-earth magnet according to claim63, wherein the outer protective layer further contains an inorganicadditive.
 121. A rare-earth magnet according to claim 64, wherein theouter protective layer further contains an inorganic additive.
 122. Amethod of manufacturing a rare-earth magnet according to claim 86,wherein the magnet body is heat-treated in the protective layer formingstep in an oxidizing atmosphere containing an oxidizing gas whileadjusting at least one condition of a partial pressure of the oxidizinggas, a treatment temperature, and a treatment time such that theprotective layer has the first layer and the second layer.
 123. A methodof manufacturing a rare-earth magnet according to claim 86, furthercomprising a pickling step of pickling the magnet body prior to the heattreatment.
 124. A method of manufacturing a rare-earth magnet accordingto claim 86, wherein the oxidizing atmosphere is a steam atmospherehaving a steam partial pressure of 10 to 2000 hPa.
 125. A method ofmanufacturing a rare-earth magnet according to claim 86, wherein thetreatment time is 1 min to 24 hr.
 126. A method of manufacturing arare-earth magnet according to claim 93, wherein, in the outerprotective layer forming step, the magnet body is heat-treated in anoxidizing atmosphere containing an oxidizing gas while adjusting atleast one condition of a partial pressure of the oxidizing gas, atreatment temperature, and a treatment time such that the protectivelayer has the first layer and the second layer.
 127. A method ofmanufacturing a rare-earth magnet according to claim 92, wherein, in theouter protective layer forming step, the outer protective layer made ofan oxide layer having a composition different from the inner protectivelayer is formed on the surface of the inner protective layer.
 128. Amethod of manufacturing a rare-earth magnet according to claim 93,wherein, in the outer protective layer forming step, the outerprotective layer made of an oxide layer having a composition differentfrom the inner protective layer is formed on the surface of the innerprotective layer.
 129. A method of manufacturing a rare-earth magnetaccording to claim 92, wherein, in the outer protective layer formingstep, a resin layer forming coating liquid containing a resin is appliedonto the surface of the inner protective layer and dried so as to formthe outer protective layer made of a resin layer.
 130. A method ofmanufacturing a rare-earth magnet according to claim 93, wherein, in theouter protective layer forming step, a resin layer forming coatingliquid containing a resin is applied onto the surface of the innerprotective layer and dried so as to form the outer protective layer madeof a resin layer.
 131. A method of manufacturing a rare-earth magnetaccording to claim 92, wherein, in the outer protective layer formingstep, the magnet body after the inner protective layer forming step issubjected to chemical conversion treatment so as to form the outerprotective layer made of a metal salt layer containing a metal salt onthe surface of the inner protective layer.
 132. A method ofmanufacturing a rare-earth magnet according to claim 93, wherein, in theouter protective layer forming step, the magnet body after the innerprotective layer forming step is subjected to chemical conversiontreatment so as to form the outer protective layer made of a metal saltlayer containing a metal salt on the surface of the inner protectivelayer.
 133. A method of manufacturing a rare-earth magnet according toclaim 92, wherein, in the outer protective layer forming step, the outerprotective layer made of a layer containing an organic-inorganic hybridcompound having a structural unit made of an organic polymer and astructural unit made of an inorganic polymer is formed on the surface ofthe inner protective layer.
 134. A method of manufacturing a rare-earthmagnet according to claim 93, wherein, in the outer protective layerforming step, the outer protective layer made of a layer containing anorganic-inorganic hybrid compound having a structural unit made of anorganic polymer and a structural unit made of an inorganic polymer isformed on the surface of the inner protective layer.
 135. A method ofmanufacturing a rare-earth magnet according to claim 101, wherein themagnet body containing an unprocessed part is pickled in the picklingstep.